There's a big difference between running a server that does basically glorified inventory tracking, and running a server that hosts (even just a wrapper around)

There's a big difference between running a server that does basically glorified inventory tracking, and running a server that hosts (even just a wrapper around) payment infrastructure. If my inventory service breaks I might have some pissed-off customers or have to re-ship some items, the odds of getting in bigger trouble than that are low. If my payment infrastructure goes down or gets compromised, the odds of lawsuits or federal/financial-org penalties are way higher. So sure, you still probably have to have a "server" somewhere (you could probably make a fully static site for a merchant using third-party payment links, javascript, and mailto links or something--think "email as database"--but I suspect this would be neither featureful, secure, nor reliable), but the difference in what you need to maintain/serve is huge.