I hate to reiterate something that has already been said, but again, it's kind of the user's responsibility to monitor usage so that you can catch it BEFORE the overages occur.  It's fairly easy and quick to check usage.

Although I did use to work for Verizon Wireless' customer service department and unfortunately many people [for whatever reason] never even think to check their usage until they've gone over at least once.  In my opinion, it's best to check your usage at least once a month, possibly a couple of days before your bill cycle runs out so that if you are going over, you can remedy the problem.