I purchased Royal Canin products from the retailer Chewy in the Canadian market and shipped the units to an Amazon warehouse. However, Amazon has lost some of my units in their warehouse, and they’ve asked me to provide an invoice. I sent them the invoice from Chewy, where I bought the products, but Amazon did not accept it.
You just need to send the receipt with the ASIN clearly marked and the proof of delivery from the carrier showing who signed for it at the warehouse. Also, send them an screenshot of their own policy where it says a receipt from another seller counts as proof of ownership.