Amazon: some items "can't be shipped to a P.O. Box", but then they ship via USPS

This has been a recurring problem I've had for many years. Otherwise, I'm a happy Amazon customer. For seemingly random items, Amazon will tell me that it "can't ship to your default address." My default address contains 4 lines, like this: My Name My Street Address P.O. Box Number City, State, Zipcode For these items, I'm forced to use an address that contains lines 1, 2 and 4 but not the P.O. Box. Amazon has tried to tell me this might be because some types of products can't be shipped to P.O. Boxes, but when I asked the Post Office whether there were restrictions on items that could be shipped to a PO Box, they said there aren't any, and specifically said that items I mentioned, such as video games and relatively small electronics accessories (such as headphones) are perfectly permissible to be sent to a PO Box. But this isn't the worst of it: Often, Amazon then turns around and ships these items via a method that requires delivery via the US Postal Service. Either t