It's all in how you appear. If you are nervous and act strangely, you will look suspicious and just attract unwanted attention. The worst part is being stopped by security at the door and get searched. Just be calm and act naturally.
I just saw a man, probably in his 50's, dressed in normal office male attire, buy a pretty flowery dress at H&M today. My first reaction is, he is just buying for his wife, who may still be somewhere in the shop floor looking for things, and he is just paying for her dress at the cashier. Nobody will think he buys it for himself to wear. No I don't know, because I haven't asked him. But nobody paid undue attention to him either.
So it's all just in your mind. Nobody cares who you buy it for.