If you will check my phone number with this account, you will see that my Go Phone balance is now approaching the magic $500 limit. This is an emergency use only phone and thus sees little use. I have an auto-refill for $25 as the account expires. Given that the next refill will take it "over-the-top", I would hope that I would be charged only for the difference between the $500 limit and the current account balance.
Dave