I shop as male or female depending on whether I have the time to dress. I'll shop in the same shops regardless.
I don't make excuses or pretend. There is no reason to tell someone why you're shopping in a particular section unless asked. If you were in male mode and shopping in a men's clothing store and someone looked sideways at you, would you volunteer that you were buying a present for your brother/father/son/boyfriend/whatever unless it was true? No. The same applies to shopping for women's clothing.
Always be polite, and don't hesitate to ask an SA for help. I've found that it's much easier to ask a SA and get help than to be a male alone in a ladies clothing store. People actually pay less attention if you're with an SA.
About the only thing I've ever bought online has been a few of my wigs. Everything else I've bought in person, and I don't remember ever having a problem with an SA.