Some women are just like that. I dated a girl who did that, but didn't hide it. She would say she was broke, get me to pay, and then buy clothes. It bothered me a bit, sure, but since she didn't hide it, I accepted that it was just part of who she was. My point is, that if this is just the way it is for your relationship, is the other stuff worth dealing with that or not. If it is a small price to pay, then let it go. If it bothers you enough, bring it up, or stop paying for stuff.
If you want proof, you could check her bin for tags after you see a new piece of clothing. If she has one in the bedroom, that is likely it, otherwise bath. If she has some shame about her shopping, she may shove them down.