What course of action do you have when your workplace doesn't pay you what they owe you?

I'm a courier and they are $135 short on my paycheck. Their excuse is that they are giving fuel charge money back to the customers in order to stay more competitive. However, no notice was given that they were going to do this. The contract states that I'm not entitled to any portion of the amount they charge the customer. The amount indicated on the pager that I've always been paid happens to be 50% but there is no mention that that price is in fact having anything to do with what they charge the customer. Do I have any kind of case here?