I ran into the odd case where I just signed up, my bill was (for some unclear reason) more than 5 days later than my billing date, and I was going to be out of town for a period. I did not want to incur any late charges and I wanted to pay my bill ahead of time. But because it was my first bill and there was no billing history and also because the limit is (I believe) 20% of your last bill as an overpayment, then I could not pay my bill in advance. If one is going to be away and not wanting to worry about bill payment during the period of a trip, it would be handy to be able to pre-pay a bill online. This might not be possible on plans that were not unlimited, but with those sorts of plans, it certainly should be. Even with the other type, you could pre-pay the expected amount with the understanding overages may occur. Any left over monies could be put towards the next bill. It's kind of ludicrous that you don't want to take my money when I want to give it to you.