It seems to me that, if the original payment terms (in British pounds) are still posted on the Internet, you can simply calculate the buyer's fee, add it to the final bid, and send an e-mail to the auction house stating that you'll wire them this amount as soon as they acknowledge that they'll send the coin to you upon receipt of that amount. In that e-mail, I would also add that if the auction house is unwilling to honor the payment terms as posted on the Internet, they have breached their agreement and you will cancel your purchase. You may lose a coin this way, but I wouldn't hesitate to cut off an unscrupulous auction house if they refuse to honor their written terms.
Glad to see that they didn't try to continue to swindle you. I've heard from others that they have done the same when demanded (resent corrected invoices in GBP). Vote with your business in the future, Brother Carausius. Don't buy from these jokers. I'm tempted at times by their wares, so I get it. They don't deserve good customers like you!
This is a luxury that some collectors just don't have. I've purchased from this firm before, as they're one of the few that semi-frequently lists coins I collect. My options are to either bid on them and pay their high fees, or let a dealer bid on them and pay his premium a month or two later, which of course incorporates those same fees. That being said, if I had won something in this sale, and it wasn't a coin I desperately needed, I'd either try to pay at the current rate or just walk away.
This is where we disagree. The other choice is to ignore the coin an tell the dealer that did buy it why you are not buying it from him. If the coin is more important than the principle, your decision has been made for you. That is the way I felt when I bought a very special coin from a certain eBay seller known for his high ratings. I still regret buying that coin. I have been collecting coins too long to have desperate needs.
He comes across as a twenty-something year old former used car salesman who is now selling ancient coins for a living. When this self-appointed "expert" describes his coins, you can tell he has no real passion for the history behind them. His "discounted prices" are outrageous, and I hate that stupid logo with his face on a coin that's on his website. Why do you buy from him?
I avoid him and I wish I could filter his over-priced low grade floor droppings out of my eBay feed. He'll rip you off if he can get away with it, and settle for reasonable value if he can't. An auction house is in the same category of it writes invoices in whatever currency suits them on the day. We already pay far too much in auction fees; have some respect your customers please.
This issue may not be as black-and-white as it initially seems. After all, you don't divorce your spouse just because he/she forgets an anniversary occasionally. (Well, some of us don't.) You don't disown your children because they make mistakes and bad decisions. Every business makes mistakes every day. Every business occasionally hires an employee who is unscrupulous, unethical, or just plain incompetent. The issue is how you deal with these mistakes. Do you acknowledge them, try to fix them, and try not to repeat them? Or do you ignore them, or worse -- pretend to pay attention to them but go on doing business as usual? I would continue to do business with a company in the former category, but not the latter category. But just how do you recognize which category the business falls into? I'm not purporting to know the category into which this auction house falls. I do know, however, that if it falls into the first category -- someone with whom I'll continue to do business -- the first step would be to allow its customers to pay in British pounds, as the terms of agreement specify. Even better, it would pro-actively (a term I usually despise) contact all the winning bidders and inform them that they can pay in British pounds and not Euros. Whether or not this auction house is in the "good" or "bad" category isn't necessarily something that can be determined from one (albeit somewhat unethical) mistake. If the members here want to give them the benefit of the doubt in this case, it's perfectly understandable.