On the old prepaid plans, each top-up amount came with a certain number of days of service, but once you'd topped up $100 you got "gold rewards" and a top-up of any amount extended your service by a year.

I'm not surprised to see T-Mobile engaged in dodgy billing. Years ago they charged me a $200 early termination fee for a plan that had a "month-to-month" contract and refused to budge even after I sent a screenshot. Their "executive resolution" department sent me a check within days of receiving the complaint from the FCC. After that fiasco I ported my secondary number from T-Mobile prepaid to Google Voice.