You must record all calls to Sprint or they will not help you. If you have a recording, tell them you are recording for quality control and to make sure they get your order or credits done correctly. You can call the sprint executive office and play the recording and they will credit your account usually.
If they don't. Then pay the $20 and file a Small Claim in Small Claims Court where you live. Because Sprint sold you the service at your location, they are required to travel to your local Small Claims Court, as the court will mail Sprint their court date to meet you in court. If they don't come, you automatically will win, and the judge will determine the case. You must bring the recordings of everyone you spoke with at Sprint saying they will credit you or fix the problems, for the judge to listen to, plus bring all your statements.
The recordings are the only way to win with Sprint. Otherwise they will never fix your bill.
You can buy a small recorder online for around $80 by Olympus and save the recordings to your computer. A recorder will pay for itself in no time. I now record all calls for everything I buy, especially the utility companies, cable companies, satellite TV etc. They all will overcharge your bill, but if you have a recording of every call to the company, then the judge will believe you and you will win.