Hello Jill,
Yes, it gets easier and easier until it's totally fun. Let me comment on two things:
This was the order you related these experiences, but I'm guessing you were feeling this nervousness the whole time you were shopping, before the incident in the checkout line. That nervousness absolutely goes away. It goes away with experience and nothing else, but it goes away much faster and easier with tips and encouragement like you get here in this forum.
Nervousness shows. It's quite possible that the customer in line picked up on your nervousness, read it as a sign that you considered yourself as doing something less than respectable, maybe even--perverted. Learn how to replace that nervousness with confidence and you won't believe how much less frequently you run into incidents like this.
Happy shopping!
Sonia