Customer abuse & Amazon refunds

Hello! Last year we received 5 orders from Romania for refurbished iPhones each costing 1000+ eur. Customers receive them and then tell that we sent them shoe wax and other items telling that we are scammers. We had sold over 600k worth of devices last year, but despite help messages and A-Z claims Amazon refunded the customers. We also provided proof from DHL that package was not tampered with and the weight corresponds to sending iPhones as we are.

Today another refund was made to a customer who sent us back a iPhone lookalike toy that had a metal plate inside to match the weight. We asked help and appealed twice. How can we get help so these things won't happen????