Amazon requires that all new listings have a GTIN (Global Trade Item Number), a unique product identifier for each product variant that is used to create an ASIN(Amazon Standard Identification Number). When you’re creating a listing through the Amazon sales channel, then you can add a GCID, UPC, EAN ASIN or ISBN to your product variant information to fulfill this requirement. The product identifier you add will automatically be included when you create an Amazon listing. In Shopperr , upload the UPCs you bought on “Marketplace>UPC maintenance” page, then when you export the product information from “Subscribed” page, choose “yes” on the instruction top-up to “match upc” , Shopperr will match the unused UPCs automatically into the product information sheet.