I certainly can't see anything wrong with your letter. If you are convinced that it falls within the guidelines as specified below, I'd be questioning AP's decision and asking them for an explanation. Although it's not a particularly large sum of money there is still a principle involved here and it's wrong of them to charge excess postage without at first giving the sender the right of reply. I note that you had you return address on the envelope so why did the recipient have to pay the extra when the request should have been sent to you instead? I thought that was the usual procedure.