Surely to make it worthwhile shipping from Europe through a 3rd party, a fair sized stock would have to be maintained in order that people can have what they want, when they want it. I have just done 2 orders to EU companies and as far as I can understand it---if the order (including carriage AND LOCAL TAX) is under £135, the supplier charges the local tax and just posts the goods to you. If the order is OVER £135, then the supplier does NOT charge the local tax, and it goes through the UK's customs system and one gets charged UK VAT, a small 'duty' and a £12 handling fee.
This seems fairly straight-forward UNTIL the parts you are ordering come originally from outside the EU and then the whole system gets complicated--very complicated!