That is how retail works. You go and buy a hundred thousand of something from a company (in this case USPS) and then sell it to consumers. Your profit is the discount you were able to negotiate by making a bulk order.
Correct. The USPS has what are called "published" rates, which is the set of prices that can be shown to consumers. Labels marketplaces (like Etsy) get an unpublished discount on top of those base rates and that's where the profit comes from. Those contract rates are not disclosed.