I had the same thing happen (I can tell we both received the same form letter). In my case I sent an email to the buyer (this happened 4 months after the sale) reminding him he agreed to pay these fees. Fortunately for me, he had already paid them (ie 3 days before he received my email), and I never heard again from FEDEX.
My policy now is the same as Cytocycle's - USPS or nothing.
My policy now is the same as Cytocycle's - USPS or nothing.