What is the legality with Amazon?

I have been selling Amazon for a year now. I focus on online and retail arbitrage. I recently have been coming across with issues with Amazon losing items for my FBA shipments by the discrepancy of items U sent to what they counted. Specifically with Amazon not accepting popular retailers invoices or receipts to reimburse me to proving the amount of items I sent in. My issue comes where Amazon being so inconsistent as I have used the same retailer receipts before to be reimbursed and everything went smooth, but here and then it becomes an issue. They repeat automated responses back about showing supplier address, my address, quantity etc.. which all have been outlined for them to be easier to the reader to be seen and still they deny it. It has gotten to the point where I take it as a loss.

My question is, how can Amazon get away with this? Before anyone mentions that you should not be doing Online or Retail arbitrage and expect them to reimburse is absurd. I would argue that a grand majority of sellers do the same as me, along with Amazon turning a blind eye towards it as well even if they frown upon it. In fact, I would argue it is more so encouraged to do so.

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My anger more so comes to me that I sent six products, in six different boxes, with six different tracking numbers all showing delivered to Amazon facilities yet they only counted 4 out of the 6 tracking’s I sent. It is clearly obvious six items arrived to Amazon and yet they refuse to reimburse me or at the very least return my package I sent them. So what does Amazon do to all these lost items from situations like myself? I am aware of it being a constant issue of items being lost. Amazon refuses to reimburse me. Refuses to send my box back that clearly shows it got delivered. How is this legal at all?