I am coming from a different angle on this one, you know you owe them something and you do, that is a definite. I would send a recorded deliver letter to Frederick Smith, chairman, president and chief executive. Telling him your problem and asking him to reimburse you for the cost of postage of the letter. You should write the words Chairmans Complaint on both the envelope and the letter, this will get an immediate response.
It is a bit of a peculiar amount as normally there would be VAT as well as duty and clearance to pay, which as you know would have made the amount higher. Duty varies on the product you bring in, the one I do carries a duty of 3.4%
I use DHL most times for my import carriage and they charge £10 for the work of handling clearance, to be honest I do not mind this amount and it seems fair.
I hope you get things sorted quickly.