Worry not, Tommy. Worst case scenario is that you get clobbered for VAT, Import Duty and a tenner handling charge. There'll only be one handling charge as it'll be a single box, so given that you got them for half the UK price you'll still be ahead even if you have to pay all these.
On the other hand you may get lucky. I just have - I bought two mouthpieces from the States and they got delivered without any payment from me.
One odd thing has happened a couple of times. I've imported stuff and been charged VAT but got away without any import duty. Dunno how that works - if I'm assessed as liable for VAT I should be paying duty too, but I have never queried it.
Good luck anyway, and enjoy your new mouthpieces.
lol, Fokker was dutch, but made the things for the Kaiser. And Fokker translates literally into english with the altered spelling. But without being a swear word.