If you've got £126 of parts without getting charged by customs then you've been very fortunate, as the threshold for them to charge VAT is just £18. I'd have expected you to be charged 20% of the £126 (if that was declared on the cusoms form), plus £10.50 wasted on FedEx, so £35.70.

I had one recently where the VAT I owed was about £3, which was fine by me, except I also had to pay Fedex £10.50 which doesn't seem fair.