it also sounds as though transactions are still in dollars not euros
What do you mean by that? Money is convertible...
Oh, I'm always interested in how my money leaves my bank account!
The banks here in the UK can and will charge a small commision rate for the exchange. I applied for 2 USA visas not so long ago and my bank (Natwest) charged me for the commision of using my card to order in US Dollars.
Banks in Australia also charge a commission for foreign currency transactions (for example, my bank charges 3% commission) and the amount paid varies depending on the exchange rate on the day. I assume Nu-Knees' bank does something similar. If the European Supply Store had the option of paying in Euros (or GB£), it would eliminate that issue for a lot of customers. But, alas, not me, unless I can somehow pay in Australian dollars!