Goodwill requests should be based on maintaining a good relationship with the creditor, not on who is right or wrong.
So rather than writing the creditor and claiming you weren’t late, or admitting that you were… simply write them and tell them how long you’ve been a customer, how much you like their service, and how you’d really like to remain a customer of theirs if you can. Then ask them if they will remove the late payment as a gesture of goodwill.
Your letter might say something like this:
I’ve been a customer of yours for years now, and recently when I was reviewing my credit reports I noticed a late payment from back in June of 2010. I have paid my bills on time, every time, for as long as I can remember. I really believe that this late payment must have been due to an error of some kind, such as an issue with the mail or something similar. If you’ll review my account history you’ll see that I have never been late aside from this instance.
My credit is important to me and my relationship with your company is also important. I’ve been a happy customer for many years, and would like to remain one for many more. Based on our good relationship, I’m asking you to please remove this late payment from my account and from my credit.
John the Consumer"
Obviously, this approach works best when you have only a single 30 day late payment with a creditor. It’s a great tool to have when your payment history is otherwise clean and you just have a "slip up" here and there with different creditors.
You don’t necessarily have to write the creditor for this to work. I personally have done it once myself with a single phone call. If you can be polite on the phone and have a good history with the creditor, it’s certainly worth a try. If the phone call fails you can always write them afterward and try again.
While the goodwill request tactic isn’t exactly a "high power" credit repair tactic, it’s still a valuable one, as it is a perfectly legal and ethical tactic that anyone can use to remove late pays from their credit reports.