Try just picking up a small item at the grocery, like a lipstick or an eyeliner pencil, and mix it in with your other purchases. Most of the time they'll just ring it up. Occasionally you'll get the "eye", and you have some options at that point. A) Yes, its for me B) You consult a list that you are carrying and say - yes, that is what she wanted, or C) Feign deafness, smile a lot, and pay (what I do). I find that you'll get this more with young female cashiers, so you may want to do a "survey" and pick your cashier.
After all, it's really none of their business what you buy. Don't build it up into a big thing in your mind.
You'll find that about 75% of the cashiers really don't give a rip. I even got complements on some heels that I bought at a salvation army store from the lady cashier. But I know that the first time buying this stuff is terrifying. Hang in there, sister.