I'm sure you've thought of this but just in case... When I was starting out, I rented a post office box and ordered things out of the Sears catalogue. Now, with the internet, things are even easier. The only problem with mail order can be sizes. Not all size tens are the same. Dresses are easier than pants or blouses.
If you decide to go to a store, sometimes enlisting the help of a sales clerk (not easy in some stores!) can give you the cover you need. Describe the garment you seek. It was a great idea to mention your girlfriends hair color and gift receipt...I agree. Genius.
Ultimately, the best thing for me was finding a female friend who would be understanding to shop with me. She was the cover and I got to try on things in the booth. Made getting the proper size a heck of a lot easier.
Like you all, I couldn't wait for each Halloween. Even better was Christmas. I mean -- It's EXPECTED that you are buying gifts for other people!