Though I guess there must be some sort of control since the station is depleting its stock of gas, so it's not like they can order more gas and say they didn't sell what they had... (or can it?)

Which is a different case from doctors and dentists who'll only accept cash or charge more for any other form of payment that is not cash (including the ones that don't have fees).

Which is the reason excess exercising is also bad for you. Think about sport injuries involving ligaments, stress fractures, even heart attacks. Of course, that's more common for professional athletes who are contract-bound to train many hours every day. But even if you lift weights: the reason training regimens make you cycle through different exercises is to allow some muscles to rest for a couple of days even if you hit the gym everyday.