When you ask them if they want a bag, don't focus on if you're going to stutter on the word bag, visualize giving them the bag. Don't avoid asking anymore. The fear is just building up and making it worse. If you're really bold, you will give them eye contact while saying bag. I think in a couple shifts you'll have an easier time saying the word. If you have a bad block, remember to be proud that you are not avoiding the word anymore.