It’s always fun when people respond to what they think you are going to say, rather than what you actually do say. These are a few of my favorites:

Round 1:
Me: Stop again soon!

Customer: No thanks, I’m fine.

Round 2:
Me: …and you’re all set.

Customer: No thank you.

Round 3:
Me: Need a bag today?

Customer: No thank you! I got an electric car! *chuckles*

Round 4:
Me: Is that all for you today?

Customer: No thanks.

Round 5:
Me: Need a bag today?

Customer: No thanks.

Me: Okay, you are all set then!

Customer: Can I get a bag?