Your creditor does not have to waive a late penalty, but they might if you have been a good customer. Being a good customer means that you have had an account with the creditor for a while and that you have usually paid all of your bills on time.
The Fair Debt Collections Practices Act gives you the right to send a letter to a collection agency telling them to stop contacting you. The letter can also point out billing mistakes and request verification of the debt (such as a copy of the contract that you signed). If the collection agency does not follow this request, you can sue them.