Your add-on starts once your monthly allowance is used up. If you buy a data add-on,
it starts being used after your monthly data allowance and Data Rollover are used
up. A 200-minutes add-on starts once your monthly minutes allowance expires.
Make sure you buy the add-on when your allowance is close to running out. A 7-day
data add-on will expire exactly seven days after you bought it, not seven days after
it starts being used.