Back in Stock Email on Shopify: One Setting, Two Vendor Readings

A back in stock email is an automated alert sent to shoppers who asked, on a sold-out product page, to hear when that item returns. On Shopify the send keys off tracked inventory, so one store setting, Continue selling when out of stock, silences Klaviyo’s alert and can trigger Omnisend’s.

Both vendors say so in their own help centers. Klaviyo’s back in stock form article says that with the Shopify setting on, “the product always appears available, even with zero inventory”, the Notify me button and form do not appear, and “No back in stock emails will be sent, since Klaviyo cannot detect when inventory is restocked to trigger the flow.” Omnisend’s back in stock article answers the same question the other way: with the setting on, “Omnisend treats that variant as available for purchase”, and “Back in Stock notifications may be sent” to the people waiting. This guide reads Klaviyo’s, Omnisend’s and Shopify’s documentation as served on October 19, 2026. It is published by Relvino, which decides per shopper across email, SMS and pop-ups and so has an interest in how stores message their shoppers; read it as informed but interested.

What “back in stock” means to a Shopify store

For a shopper, back in stock means the thing they wanted can be bought again. For a merchant it is narrower, because Shopify defines the opposite state precisely: “An item is out of stock when inventory is tracked and the inventory level is at zero or below.” Back in stock is the moment a tracked variant’s count rises above that line, and the alert tools listen for exactly that change.

Two consequences follow. An item whose inventory is not tracked is never out of stock in Shopify’s sense, so there is no sold-out state to watch. An item allowed to oversell is never unavailable to buy, even at zero. Shopify lists the reasons a store might allow that, and the first is “You have products available for preorder.” That first reason is why this one checkbox matters for back in stock alerts.

One Shopify setting, two vendor readings

Take a jacket that sells out in size M with 140 shoppers on the waitlist. The merchandising team turns on Continue selling when out of stock so the size can be preordered against a shipment due in three weeks.

On Klaviyo, the waitlist goes quiet. The product page now shows the variant as available, so the Notify me button disappears, and while the setting stays on Klaviyo cannot detect a restock, so no back in stock email goes to anyone on the list, even after the shipment lands. Klaviyo’s instruction is direct: make sure the setting “is disabled for those products” to use its back in stock feature.

On Omnisend, the waitlist may hear about it while inventory is still at zero. Omnisend explains why: “This happens because the Back in Stock feature is designed to notify subscribers when a product becomes available to buy.” A preorder run through this setting makes the variant buyable, so a subscriber can get a back in stock message for an item that will not ship for weeks. Omnisend offers two ways out: disable the setting and restock normally when ready to notify, or pause the automation before turning the setting on.

Neither behavior is wrong. Klaviyo reads back in stock as restocked inventory; Omnisend reads it as available to buy. Either way the vendor’s reading, not the merchant’s intent, decides whether the waitlist hears about a preorder.

How a back in stock email works, step by step

On both platforms the mechanism has three steps: a sign-up, a wait tied to one variant, and a send when inventory returns.

1. The sign-up on the sold-out page

Klaviyo places a Notify me button on any product page where inventory is out of stock, and “The button appears and disappears automatically based on your product inventory.” The button can replace the Add to Cart button or sit beneath it, and an account can have only one back in stock form at a time. A shopper who clicks it chooses email, text or both, depending on which fields the store put on the form. The two channels carry different consent rules: an email alert can be requested without joining the marketing list, with an optional marketing checkbox, while a text alert requires consent to promotional texts, and Klaviyo says “This consent is required and cannot be made optional”.

Omnisend’s form must be targeted to show on out-of-stock product pages, and it treats email consent much as Klaviyo does: “By default, contacts who submit the form are collected as non-subscribers.” Adding a Legal Consent block turns those who tick it into subscribed contacts. Either way, the sign-up records interest in one variant, not the product. Omnisend names a variant mismatch as the most common reason an alert never arrives: “Customers must subscribe through the exact variant URL that went out of stock.” A shopper who signs up on the small while waiting for the medium is never told the medium is back.

2. The wait

In Klaviyo the sign-up logs a Subscribed to Back In Stock event, which triggers the flow, and the shopper then sits at a back in stock delay, a step unique to this flow, until the variant is restocked. The waiting list only fills if the message after the delay is set to Live or Manual: “If the message after the delay is set to Draft, no one will be put in the waiting list.” A flow built in advance and left in draft collects no one. Omnisend tracks the wait by product and variant ID, so “Back in Stock workflows rely on product ID and variant ID, not the product title.” Renaming a product does not lose the waitlist.

3. The restock and who is told first

When the count rises, Klaviyo’s settings decide who moves on. Out of the box, “By default, all subscribers are notified at once when an item is restocked.” Two account settings change that. A restock minimum sets how many units must come back before anyone is told. A notification rule sets how many subscribers are told per unit and how long to wait between batches.

Two details in Klaviyo’s documentation are easy to miss. The settings live at the account level, so “changes made here apply to all back in stock flows in your account”; the same rule covers a limited drop and a basic tee. And for a store using Shopify’s point-of-sale hardware alongside its online store, “the back in stock flow will count all of the inventory in your physical store as well as in your warehouse.” Units received at a shop can count toward a restock.

When the waitlist is bigger than the delivery

The harder case is a popular variant with more people waiting than units arriving. Klaviyo’s worked example: 20 units restocked, with customers to notify set to 5, means “5 customers will be notified per unit, resulting in the oldest 100 customers being notified.” The rule is per unit, and the order is by age on the list: the earliest sign-ups go first. After the wait time, Klaviyo checks inventory again and sends the next batch, and “Emails will continue to be sent in batches until the existing inventory drops below your account's minimum inventory threshold.” If more stock arrives mid-wait, “additional emails won't be sent until the end of the waiting period.”

Batching protects the back of the list, not the shoppers already told. Klaviyo says “it's not possible to check if the item is still in stock before sending a second message”, so a reminder sent later in the flow can point at an empty shelf. Klaviyo’s own guidance keeps the flow short anyway: “Typically, you will only need a single message in this flow”, with Smart Sending turned off for it so that everyone on the list gets the alert.

Omnisend’s back in stock article, read for this guide, describes no restock minimum, batch size or notification order; its stated controls are the form, the automation and the Shopify setting above. For drops that sell out in minutes, how a tool orders and paces the send matters as much as the email, and Klaviyo’s default is everyone at once.

Klaviyo vs Omnisend back in stock, side by side

Quoted phrases are from each vendor’s own help center; every other cell is this guide’s summary of those pages.

• Stores that get the built-in form · Klaviyo: Shopify and BigCommerce (the flow also runs on other platforms through a snippet or API) · Omnisend: Shopify, BigCommerce and WooCommerce
• Shopify’s Continue selling when out of stock is on · Klaviyo: No button, no form, and “No back in stock emails will be sent” · Omnisend: “Omnisend treats that variant as available for purchase”, so notifications may be sent
• Who is told at a restock · Klaviyo: Everyone at once by default; optional restock minimum and per-unit batches, oldest sign-ups first · Omnisend: Not stated in the back in stock article
• Email sign-up consent · Klaviyo: Restock alert without marketing consent; marketing checkbox optional · Omnisend: Non-subscribers by default; a Legal Consent block makes them subscribers
• Product name in the SMS · Klaviyo: Catalog tags insert the restocked item · Omnisend: No dynamic event data in SMS; a general line instead
• Demand report · Klaviyo: Back in stock reports; owners and admins can export queued subscribers as CSV · Omnisend: Back in Stock Requests report; no export

What a good back in stock email says

The shopper already chose the product, the size and the color; the email’s whole job is to confirm that exact variant is buyable and get them to it in one click.

• The variant, not the product. Name the size the shopper signed up for, such as the Linen Overshirt in M, show that variant’s image, and link the button to the variant, not the collection.
• Current details, pulled at send time. Klaviyo builds this in: its alert template uses special variables that “look up the restocked product in your catalog at send time and populate the most up-to-date details available”, because a price or photo may have changed during the wait.
• An honest stock line. If the restock is small, say so plainly; if the store batches, the first batch genuinely is first in line.
• One message, sent once. The shopper asked for one alert. A second message cannot be checked against current inventory, as above.
• A text that stands on its own. Omnisend says a back in stock SMS cannot use dynamic event data such as specific product names and suggests a general line instead. The email has to carry the specifics.

Where the alert sits among a store’s other emails

A back in stock email is the rare automated message a shopper explicitly requested. A browse abandonment email guesses from a product view; an abandoned cart email guesses from a cart. Inventory runs through those flows too, in reverse: Klaviyo documents a template change that skips an abandoned cart or browse email when an item in it has sold out, logged as “Skipped: Catalog Item Unavailable”.

Once the restocked item is bought, the shopper moves into the post-purchase emails like any other buyer. The ecommerce email automation guide maps how these messages fit together, and the email marketing examples hub describes a good one of each type.

Decide what “back” means before the next sell-out

Several of the failures above are decided before the first alert: which variants track inventory, which ones oversell for preorder, and which vendor reading of “available” the store is running.

• Write the preorder policy into the alert plan. On Klaviyo, a variant preordered through Continue selling when out of stock has a silent waitlist; on Omnisend, it can trigger the alert. Decide whether a preorder should reach the waitlist, then use the Shopify toggle, and on Omnisend a pause of the automation, to get that result.
• Set the restock minimum and batch rule before a drop, not during it. On Klaviyo they apply to every product in the account.
• Put the message live before the form. A draft message means an empty waitlist on Klaviyo, and both vendors require the automation to be on.
• Keep the waitlist data. Klaviyo’s export of queued subscribers and Omnisend’s requests report record unmet demand by variant, which the buying team can use.

Done in that order, the alert does the one thing the shopper asked for: it arrives when the exact item they wanted can be bought, and not before.

Frequently asked questions

How do you write an email for stock availability?

Lead with the item, not the brand. A subject line such as Back in stock: Linen Overshirt, size M is unglamorous and hard to misread. The body needs one line confirming the exact variant, its current image and price, one button to that variant’s page and, if the restock is small, a plain note saying so. A discount is optional: the shopper wanted the item at full price before it sold out. Klaviyo keeps a pre-built back in stock email in its Email templates tab, found by searching back in stock.

What does “back in stock” mean?

For a merchant, it means a variant whose inventory is tracked has risen back above zero where it can be sold online. Two details in Shopify’s help center shape that. The Continue selling when out of stock option can only be selected when inventory tracking is on. And if a location that fulfills online orders is at zero or below while another location that does not fulfill online orders has stock, Shopify still displays the item as out of stock on the online store, so stock in a shop that does not ship web orders does not bring the item back for online shoppers.

How can I get notified when something is back in stock?

On a store that offers alerts, a Notify me button or sign-up form appears on the product page when the selected variant is sold out. Choose the exact size or color first, because the alert is tied to that variant: Omnisend’s embedded form displays only for the specific variant that is out of stock. Depending on the store’s form, a shopper can leave an email address, a phone number or both. Not every sold-out product has to offer an alert: on Klaviyo, a Shopify store can use product tags to include or exclude the products where the Notify me button appears.

Should a back in stock alert go by email or SMS?

Often both, in one flow. Klaviyo recommends adding SMS to the existing back in stock flow with a conditional split rather than building a separate SMS-only flow, for which Klaviyo says the minimum inventory and customer notification rules must be configured, and only current SMS subscribers receive the text. It also notes that some audiences prefer the email. Omnisend lets a brand include SMS in its back in stock automation but without dynamic event data such as product names, so its text has to be general, which makes the email the message that names the item.

Why are my back in stock emails not sending?

The common causes are in the vendors’ own troubleshooting notes. On Klaviyo: Continue selling when out of stock is on for the product, the message after the back in stock delay is still in draft, or the theme hides products whose variants are all sold out, so there is no page to sign up on; Klaviyo also says the Retina theme’s built-in Notify me form must be disabled first. On Omnisend: the form or the automation is not enabled, the form is not targeted to out-of-stock product pages, or the shopper signed up on a different variant from the one that was restocked.

How long does it take to migrate from Klaviyo to Relvino?

The technical cutover takes about 30 minutes: connect the Shopify store, point the sending domain and SMS number, and shopper data ingests automatically. Revenue is then measured in a 14-day pilot. Separately, before changing any platform, Klaviyo’s help center describes a CSV export of currently queued back in stock subscribers, with email, date added, SKU, variant ID, product title, variant title and unit price, available to account owners and admins. It is worth saving as a record of which variants shoppers were waiting for.