Back in stock alerts for a specific size or color, not the whole product

A customer wants the Medium, not the Small. She taps “notify me” on your sold-out product and goes about her day. Two weeks later the Small comes back, your app emails everyone who signed up for that product, and she clicks through to find the Medium still gone. Do that to her twice and she stops opening your emails.

That gap has a name. It is the difference between a product-level alert and a variant-level alert, and it decides whether your back in stock emails get trusted or tuned out.

Product-level vs variant-level, in plain terms

Product-level means one signup covers the whole product. When any variant restocks, everyone who ever signed up for that product gets the email, no matter which size or color they actually wanted.

Variant-level means the signup remembers the exact variant. The person waiting on Medium Blue only hears from you when Medium Blue is back, and never for any other size or color.

Product-level looks fine when you test it on a product with one variant. It quietly breaks the moment you sell anything with sizes, colors, or both, which is most stores. The more variants a product has, the more wrong-variant emails you send, and the faster your list learns your alerts are not worth checking.

Why a wrong-variant alert costs more than a missed one

A missed alert is a customer who never hears from you. That is a lost sale, and it is bad. A wrong-variant alert is worse, because it actively teaches the customer that your emails are unreliable. She opened it, trusted it, drove to the page, and got nothing. The next real alert, the one for the size she wanted, lands in an inbox that has already decided to ignore you.

There is a direct cost too. If your app charges by the alert, every email to someone whose variant did not actually restock is money spent training a customer to unsubscribe. You pay to make the problem worse.

Shopify itself treats variant-level as the baseline, not a premium feature. Shopify Flow ships a Product variant back in stock trigger, which only fires for the specific variant that came back. If the platform’s own automation tool works at the variant level, a notify-me that does not is behind, not ahead.

What variant-level actually requires under the hood

Three things have to line up, and an app can get the first one right while quietly missing the other two:

1. Capture the variant at signup. The widget has to know which variant the shopper had selected when she tapped notify me, and store that with her email. A form that just grabs an email against the product ID cannot ever be variant-level, no matter what the listing says.
2. Match on the right restock event. When inventory for that one variant crosses from zero to positive, only its subscribers should be notified. Not the product’s whole waitlist.
3. Get the multi-location math right. Shopify inventory can live across several locations. The count that matters is the one a customer can actually buy from, so an app has to read availability correctly or it fires early on stock that is not really sellable yet. This is the part that is genuinely fiddly, and it is worth asking any app how it handles it.

Test it in five minutes before you trust it

You do not need to read anyone’s feature list. You can check this yourself on a live product:

1. Pick a product with several variants and make sure one of them, say Medium, is genuinely sold out.
2. Sign up for the alert with Medium selected.
3. Restock a different variant, like Small. You should get nothing.
4. Now restock Medium. You should get exactly one email, for Medium.

If step 3 sends you an email, your setup is product-level under the hood and every multi-variant product you sell is quietly generating wrong-variant alerts. That is the thing to fix before you turn it loose on real customers. If you also want the button itself to behave, the companion piece on adding a back in stock button covers the variant-aware rendering side.

Where Kelso lands

Kelso is built variant-first. The widget captures the exact variant a shopper picks, matches on that variant when it restocks, and reads availability across your locations so the alert fires on stock a customer can actually buy. Every send is logged against the subscriber and the variant they asked about, so when someone says “I never got my alert” you can look up exactly what happened instead of guessing. Variant tracking is unlimited on every plan, including the free plan at 50 sends a month with no card.

Whatever you run, run the five-minute test above first. It proves the alert targets the right variant, and the broader ten-minute test proves your alerts actually fire and land before you rely on them. A notify-me that emails the whole product is not saving you the wrong-variant complaints. It is scheduling them. Kelso is live on the Shopify App Store if you want to try the variant-level version.