If it’s sent as a letter you should hopefully be fine, but otherwise I’d have thought you’d have customs and VAT on a £2 item which would be very little, then for an item so low value I would hope they’d give you the option of the lower online admin fee of 2€ given it was a ‘letter’ in an envelope for an item that cost almost nothing, so you could get away with 3€-ish in theory if they were to be reasonable and fair and the luck gods were shining on you that day. But perhaps that’s wishful thinking and they’ll go for the higher fee. It also depends of course on what it’s declared as at customs and how rigid they’re being with applying the process from day 1. The only evidence I’ve seen seems to suggest that at this stage they’re trying to make it as smooth as possible.
In terms of rejecting it, no there’s no comeback for you as far as I’m aware, the opposite in fact as under distance selling regulations you have a right to return, although without your proof of delivery when you send it back (which you won’t have as you obviously never accepted delivery) the seller could perhaps make a fuss about refunding you your £2 and say they never received the item back, although technically the DSRs don’t actually require you to receive the item to cancel the contract. I do however have a friend who’s had a hellish time since January selling things from the UK to EU and having a significant number of people rejecting deliveries so as not to pay the extra charges, which seems to result in weeks if not months of delays where the package just goes missing and she has to spend months arguing with the delivery service about where it is, and months arguing with the purchaser about why they can’t have their money back until it’s eventually returned, if it is, so from an ethical standpoint I’m not sure I’d reject items unless entirely necessary as it does seem to cause hassle for the seller often. But we are talking about a £2 motor belt here so it’s not like it’s a priceless artwork or granny’s ashes so ethics are minimal really