You give them your credit card info and they automatically recharge you every month (and give you a $5 discount). That's why I said $45 instead of the advertised $50. They also give an additional $5 discount for additional phones, so I only pay $40/mo for my wife's phone on the same plan.

EDIT: just checked my bank, and it looks like after taxes and fees, the actual numbers are $49.99 and $44.50 - so while the advertised price is slightly false, it's not as bad as most of the carriers.