I could have refused, and I would have been in the right. But this seller had HTF country-specific merchandise, they originally only sold within their own country and I'd had to ask them if they would sell to the US, and if I'd refused to send the 3 euros then they probably would have decided American sales were too much trouble and gone back to only selling within their country.
This situation has happened to me before as well. Sadly when you are operating in a circumstance where you are dealing with an own-country only seller, you often have to make decisions like this in the bigger interests of getting the item you are after. I've had many experiences with sellers who refuse to sell abroad because of one or two incidents with buyers somewhere in Europe which for some reason means every other person in the whole continent is troublesome...there are occasions where delicate negotiation is necessary if it's a really important item. And that can mean being willing to stump up extra on shipping. I don't agree with sellers who cheat on shipping cost and I don't agree with sellers who make it harder or more expensive for foreign buyers because they assume that either they are entitled to be compensated additionally for taking it to a PO or that we are all scammers and thus need to have everything sent with the maximum insurance, not just tracking....but if the item is worth the hassle and I can't get it anywhere else, sometimes I will bite the bullet. I usually contact the seller, though, and ask them, making it clear I am well aware of potential shipping costs. So it's not usually the case where a seller adds on after the auction, but gives me the eyewatering priority quote during it...
I also had one auction where I contacted the seller, told them outright that I expected it to cost a high amount to ship to me in the UK and that I was ready to pay that amount,and that worked. The usual excuse people use not to ship abroad is that buyers complain about shipping costs, and sometimes that is true and can be got around by stating clearly from the start that you are prepared to deal with international shipping costs. I had one lovely seller who had never sold outside of the US but between us we worked out how she could do it and I sent her the shipping money and it worked out a little less but I told her to keep the extra for all the work she had put in to get this pony to me, even though she had no idea how to ship abroad before I spoke to her. It was a MOC item and it arrived in beautiful condition so I don't regret that I maybe paid $6 more on shipping than the actual cost. To me that was worth it.
But to the same degree it can also be an opportunity for sellers to try to exploit foreign buyers by adding on cost because they assume the 'foreigner' doesn't understand the currency/postage system and can make up things to explain the extra charge. And I am absolutely not okay with those people.
In this instance I feel the seller's behaviour was shady and that as a rule of thumb it's not a good idea to pay more than the quoted price. I have argued it out with sellers in the past too because as a seller if I misquote on shipping, it's my problem, not my buyer's problem - but not every seller has that perspective and in my opinion this seller is actively committing fraud by charging extra that is not merited outside of auction.