How the checkout block works

Easy Stock Buffer enforces your thresholds with a Shopify cart validation function - code that runs on Shopify's servers every time a cart is evaluated. Because it runs there and not in your theme, the block applies on every checkout path, including Shop Pay and express wallets, and needs no theme edits.

What it checks

For each line in the cart, the function looks up the variant's threshold and its current stock, then compares. A line is blocked when the stock left after buying it would fall below the threshold. The shopper sees a short message and cannot complete checkout until they lower the quantity or remove the item.

Where the numbers come from

The app stores each variant's threshold and a fresh stock snapshot on the variant itself. It keeps the stock snapshot current by listening for Shopify inventory updates, so the function always reads an up-to-date number. Which threshold applies follows a fixed order of precedence, covered in how thresholds decide what gets blocked.

Cart versus checkout

On the storefront cart, Shopify treats a validation message as advisory - the shopper sees it but can still adjust their cart to fix the problem. The order is stopped at checkout, so nothing over your buffer can be placed. Headless and Storefront-API stores reject the over-limit change at the cart step as well.

The rule has to be switched on once before any of this happens - see activate stock blocking in your Checkout settings. You can reword the message shoppers see in edit the checkout messages.