Hi,
This is Joy V from Shopify Marketplace Connect support. If your Amazon professional seller account is connected to the Marketplace Connect app and your products already have valid barcodes assigned, simply ensure that the Offer status of your products on the app is set to Active. Enable the “Publish Listing Details” option, assign them to the appropriate category, product type, item type, and, for multivariant products, a variation theme. Finally, click the Save button to initiate the listing process and push the products to Amazon.
Feel free to contact our 24/7 support team via chat or via email at marketplace-connect@shopify.com so we can further assist you in listing your products to Amazon.