The prices are well-engineered to suit them and their processes. But they will penalise you for stepping out of line and not moving stock quickly.
I'm sending in shipments multiple times a week to them and the logistics costs are getting higher to get stock into the hubs as they are expecting drivers to wait longer.
Some customers find solace in mixing FBA and FBM still to maintain some control.
But they own the customers. So what's that worth...
Just make sure they don't circumvent your supply chain if you sell well as they are very cute at taking you out of the chain.