I asked our postmaster about this when our mailbox was crammed full of Amazon orders . When the mail carrier came by later, our daughter's medicine , and all our other mail, was held until the next day to be delivered. She had gotten a text that informed her the med was 'out for delivery'.
The postmaster told me that Amazon deliveries were now a contractor for the USPS and were permitted to use a mailbox for delivery. And yes, the Amazon guy took a picture of our mailbox when we traced a couple of the orders back.
He told me UPS was a contractor for the USPS on priority packages as well.