Two things.
1). Regardless of what a rep says, ALWAYS re-check your plan after any changes have been made. If you do that, things like this don't happen. It's no different than checking your credit card statement every month to be sure that new $20 gym membership isn't really $50
2). Call VZW and explain the situation. It is your word against theirs but in most cases if you've paid your bill on time and this is the first or second time you've had a discrepancy with VZW, they'll credit you the charges.
Thats my $0.02
Very true. My last phone was a Voyager and I had the $15/month data plan. I wanted to get the FLO TV service (an extra $15/month) when it became active in San Diego. There was another data package for $25/month, which included the FLO TV. I asked if the change to the other data package would be automatic when I activated the FLO TV service and was told that was true. It wasn't true. I went in to the Verizon store and explained. It was changed right away.