Like this: you go to the company's website and send them 100 BC. Then, they give you an ID. They have plenty of time to get confirmations to build, and when you need a beverage you just use the ID they issued you and they debit it from your account.
a prepaid account with the snack machine company works great for snack machines in corporate cafeterias (or other places with a fairly static consumer base) not so much for areas with a large transitory consumer base, such as airports, ferry boats, train stations etc.