I guess I've just been relying on Prime too much for the last year, but I wonder: is Amazon relying on USPS more often or are other sellers on Amazon's site rel

I guess I've just been relying on Prime too much for the last year, but I wonder: is Amazon relying on USPS more often or are other sellers on Amazon's site relying on USPS? I ask because, as an infrequent seller, Amazon's shipping credit barely covers USPS First Class for the Standard Shipping option that most customers choose. I certainly can't afford to lose money shipping on UPS's slowest/cheapest offering instead.