Don’t have a UPC?
Listing the product without UPC was Big NO for Amazon sellers and availing a GTIN Exemption process was a headache.
To improve your experience, Amazon made an easy process of providing proofs for availing GTIN exemption.
Instead of any supporting letter from the Brand Owner, now seller can exempt for GTIN by simply providing Product Name, Images showing all sides (6 sides usually) of product packaging.
Reference: Amazon Product Listing using GTIN