I'm thinking that the first bill would probably be generated after the buyer's remorse period.

It's not -- I just signed up for a postpaid plan on T-Mobile yesterday, and the receipts included the first and second months' bills (for service and taxes).

If you do it on the web they make it very plain that $0 is due now. They do say the plan will be $45/month but there is absolutely no indication what their additional fees will be nor what the taxes will.

I'm in California. Maybe you are in a state where they actually have to provide this information?

I'm in Oklahoma, which traditionally doesn't have much in the way of consumer protection or information laws. It's possible, though I think it's probably because I bought it in a physical store.