I have a similar but different situation:

A couple of weeks ago I received a letter from a car rental company. They told me that they checked their books and that they forgot to bill me one rental more than a year ago (15 months!!!). As I rented a lot back then I didn't even notice.

Now they "asked" me to give them the permission to charge me. Legally I don't have to pay because it's so long ago.

The other replies above convinced me now that I should let them charge my credit card, but now I can't find their letter anymore...