It sounds like the Boost plan is similar to Virgin Mobile (which I current have/use). I can't seem to find the $10 or $20 every 90 day amount posted anywhere on their site. Anyone have a link?
Add money to your account whenever you need to cover your calls and any extras used. Recharge at least once every 90 days to keep your account active (after 90 days any credit balance will expire). Once you hit a zero credit balance, you have 60 days to add money and keep your service active and account open.
The minimum amount you can add to your Boost Mobile® account at any given time is $10 and the maximum is $100. When you see 'Re-Boost® for as low as $10' at a retailer, you can instantly add between $10 and $99 to your account at the register. Note: When using Re-Boost® Cards specifically, the minimum amount you can add is $20 and the maximum is $50 (cards come in $20, $30 and $50 denominations).