Nobody's staring. If you have long hair and no binder, you're basically presenting as 100% female in androgynous/masculine clothes. So people think you're a girl who likes to wear guys' clothes. Or maybe that you're shopping for your boyfriend. Either way, nobody really thinks twice about it. If you don't pass, use the women's dressing rooms. Like I said, butches and tomboys do it all the time. It's actually a bit more awkward when you do pass enough to be ambiguous, but choosing a dressing room is a pretty emphatic statement "I am X."
I bought jeans this week. Men's jeans. I went to the Levi's store, where the men's is on a whole different floor from the women's. I presented as male, was read as male, tried on my jeans in the men's dressing room. And then I paid for them, using my credit card in my birth name, with my ID with a long-haired smiling girly pic. Clerk didn't even bat an eyelash. This was the same store where, a few months ago, I'd bought men's jeans while presenting as female and trying them on downstairs in the women's dressing room. And they didn't care that time either.