Amazon constantly says they didn’t receive all the product that we sent them.

This has happened too many times to count, I was wondering if anyone else has this problem when dealing with Amazon. They placed an order with us, we sent all 3 boxes of product around $6k total, and we got a message today saying they only received 1 out of 3 boxes. We have proof of delivery paperwork that proves they were delivered, but they now say they don’t have it and won’t pay for the product they supposedly “don’t have”. Being a small company we can’t just let things like that slide, it’s super frustrating.