If you had a written contract, it is binding on *both* of you until and unless it is changed in writing--an oral or spoken agreement does not modify or revise a written contract, or waive any of its terms. So you owe whatever late fees the written contract says you owe, and no more than that: you have to pay whatever late fees are due based on the terms of the written contract and she cannot charge you any late fees not required by the contract's plain terms. At  need, if you cannot work any disagreements out between the two of you, either of you could file a lawsuit to enforce the contract--unfortunately, there is no other, easier, or quicker way to enforce a contract than by lawsuit.