am I overreacting?

My boyfriend and I share an Amazon account, and we have a gift card on there that we both got with our money and he wants to buy a poster. But the thing is, when we got the gift card he said we should spend it wisely on things we need, and a poster is not something we need... I want to bring it up with him, and the fact that it's both of our money and I don't want to waste it on a poster, but I don't to annoy him so now I'm just mad about it a little. Am I overreacting?