A back in stock email is the easiest sale in e-commerce. The customer already wanted the product, already tried to buy it or watched it sell out, and the only thing standing between them and checkout was inventory. When the stock returns, the email is not persuasion, it is permission. That is why back in stock consistently tops Klaviyo's revenue per recipient benchmarks, far above standard promotional sends.
Why the Native Flow Captures a Fraction
Klaviyo's built in back in stock flow depends on the customer clicking "notify me when available" on the product page. Some do. Most don't: they look at the sold out badge and leave. The buyers who viewed the product three times last week, or carted it the day it sold out, never formally opted in, so the native flow never reaches them. The opt in list is real intent, but it is the visible tip of a much larger pool of demonstrated interest sitting in your browse and cart data.
Automating the Full Audience
The inventory driven version watches Shopify's restock events directly. When inventory returns on a previously sold out SKU, the automation builds the full interest segment in Klaviyo: explicit notify me subscribers first, then recent multi-view browsers, then cart abandoners on that product, with variant level matching so the buyer waiting on the black medium never gets an email about the blue large. The send fires within minutes of the restock, while the demand is still warm, and every conversion is attributed back to the event in the log.
This is one of the engines inside our inventory driven marketing engine, where the restock trigger sits alongside overstock clearance, low stock urgency, and trending amplification, all running off the same Shopify webhook layer.