If you are charged an extravagant fee, as a result of breaching a contract, then you should refuse to make payment without getting legal advice.
When a party to a contract breaks that contract there are consequences. If the injured party has suffered damages then that injured party can recover those damages from the defaulting party but, penalties cannot be recovered.
If for instance you agree that you will pay your phone bill within a certain time, after receiving an invoice but, you fail to make that payment on the due date, then you have broken that contract. You are in default. But what damage has the phone company suffered as a result of your late payment. Well, that depends upon the circumstances. Perhaps there is no real damage suffered at all. If so, why should you have to pay a fee which amounts to a penalty?