To argue Amazon's side here, they most likely don't view this as unfair withholding. From Amazon's point of view, this seller moved from FBA to self fulfillmen

To argue Amazon's side here, they most likely don't view this as unfair withholding. From Amazon's point of view, this seller moved from FBA to self fulfillment and had a massive increase is sales. The OP said they were averaging $1M per month, and that Amazon had held $4.2M for 15 days, so it looks like they'd had an up to 8x increase in sales while simultaneously moving to a system where Amazon had no oversight. As I said, while I can feel the pain of getting automated replies from a company that owes you a lot of money, I don't think this situation lends itself to easy solutions without a waiting period, even if you involve lawyers. There are circumstances where payment processors will hang on to the cash for much longer than this. Concert tickets come to mind.