This has nothing to do with going bonkers. It's normal business practice as stipulated by the laws and regulations - if a product is not available or defective, the dealer/ vendor is obligated to give you a comparable better product for the same price. Apparently the size difference would be an issue, but you will have a hard time to prove that it's actually unusable. The only real failure here is that they could have asked before shipping anything, but they are fully within their rights to ask you to pay for a return shipment otherwise. I know it's going to be costly or you, but simply send it back and ask if they throw in a 10 Euro voucher for your next order. It's always best to settle such things with as little noise as possible.

Pimpernuckel