Barkley's Pup Store review for Walmart Marketplace

This app is a pain from the beginning. I want to move my Walmart-Shopify integration from Sellbrite to this app to simplify things and save a $20/month fee from Sellbrite. It keeps telling me that a UPC code has a minimum of 12 characters, yet my Sellbrite integration and Walmart Marketplace listings work perfectly fine with 8 characters for the UPC. Does anyone have a solution to this? I really want to use this app and not have a "middle-man" service to integrate these two networks.