Grannynannywanny
I made the mistake a couple of years ago of adding a few spare first class stamps of slightly more than the value of a European stamp to post a card to Ireland. I began to doubt myself and checked in the post office and was told it would be rejected and left undelivered as it has to be the correct European stamp.
That's not right - or at least not what the Post Office says. As long as the postage paid equals the amount due (2×first class stamps = £3.40 currently) it will be accepted. If you're not sure of the weight - maybe you enclosed a letter as well as a card - take it to the post office. You can attach an Air Mail sticker or just write that on the envelope.