That is the issue (with the admin fee). No amount was ever agreed to and no contract was ever entered into by the buyer. Surely the admin fee should be incorporated into what they charge for overseas/international deliveries as it will inevitably go through customs.
One simple letter outlining how i had never agreed to pay their £11 admin charge had them immediately waive it (i still paid the VAT due though of course).
The contract however would have been assumed to have been agreed when the buyer purchased the items and chose the shipping option.
Every single site I've ever ordered an item from when buying from abroad has tended to have the notice that "the buyer is responsible for any additional customs and clearance fees".
With that you agree to it when you click "buy" knowing the delivery method.


That was 9 years ago so might have changed.
But that's because I take offence at an £8 charge for an entry that took less than 2 minutes and I could do just as easily for nothing. And the product was only worth £18 to begin with...