Back in my grocery checker days I perfected what I called the "dumb checker" look. When a customer started being unreasonable, I just looked at them and said nothing, and just pretended that my IQ had dropped by about 50 points.

This would cause them to realize that complaining to me was about as useful as complaining to a brick wall.

When that didn't work, I called the manager. ("customer service, checkstand six please")

Remember, in retail, you are expected to be courteous to customers, but you don't have to take their abuse. That is what the manager is paid to do.