My understanding was that the free transfer is only for payments in Euros between Eurozone accounts. So Euro-GBP and vice versa incurred a cost. Either way when you make the payment you can specify all costs and currency transaction fees to be paid in advance, so the recipient gets the exact specified number of euros.
TBH, I'd go the CC route. Gives you some kind of protection, unlike bank transfer. First mistake I made here in Germany was purchasing household goods and paying up front. They have your money and mess you around and put your delivery priority to the bottom of the pile, and prioritise those paying CoD. So I always use CC or CoD now, never pre-payment for *anything*, whether it'd be bike24, rose, b-c.de, or anything else (although never had an issue with the bike stores).