Update Etsy Listing Quantities When Shopify Inventory Changes

Run whenever the available quantity of an inventory item changes at a Shopify location, from a sale, a restock or a manual edit.

Get the variant behind the changed inventory item, including its SKU.

Read the variant's Etsy listing ID from a metafield such as etsy.listing_id, which links each Shopify variant to its Etsy listing.

Continue only if the variant has an Etsy listing ID, so products you don't sell on Etsy are skipped.

Fetch the Etsy listing's full inventory, including every product (variation) with its SKU, price and quantity.

Send the listing's full inventory back with only the quantity changed on the product whose SKU matches the Shopify variant, set to the new Shopify available quantity. Etsy replaces the whole inventory on every update, so every other product, price and SKU must be sent back unchanged.