Billing issue

My bill is due on the 4th day of the month. Four days before the due date I upgraded from the 10g plan to the 18g plan. When the bill was due the 18g plan was withdrawn from my bank. I immediately went back to the Liberty 10 program. I checked and it was verified. I called to report a problem and while I had to wait 15 minutes for a service rep, I ask if the bill would be prorated on my next billing. She told me I would be charged full price for the 10g service. I had 18g for about 4 days and paid for the service for a month. Where is the $40 adjustment I should get on my next months bill. The point is, I did not use but 3g of the 18g I paid for. The service rep said I paid for the plan and not the usage. It might be in the contract, but is that fair to treat a customer that way?. Where is the JUSTICE???!!!!