When it comes to small items like most of us modelers purchase the USPS is always going to be the most economical. It’s not until you get in the range of ten pounds or more that the other two become competitive. So it stands to reason most of what we buy is going to come USPS.
It doesn’t help matters that the other two can’t deliver to PO Boxes as that is the safer option when receiving packages then on top of the mailbox as Ace said.