gregmcc: Just having a think about it, when the OP signed up for his contract was there any mention of the late payment fee? if not then they cannot vary the terms of the contract by introducing a late payment fee.
Regardless, there presumably WAS a mention that the plan cost $x/month - I hardly see how he can say 'but I don't have to pay EVERY month, just when I can' as an excuse for not holding up his side of the bargain. And a small fee appears reasonable to the majority of the above respondents as a remedy for the OP's breach of contract.