But how does thes work, legally speaking?
Sure, you have an obligation to pay the Tax and Duty.
That's a matter for you and HMRC.
But you didn't authorise the courier to pay it on your behalf, and then charge an admin fee.
How about you offer to pay HMRC directly?
( Most airports have an HMRC point ).
If you just don't pay them ( or pay only the duty ), what will they do?
Sue you? What for? Breach of contract?
They have no contract with you to enforce.
Just wondering.