@Tai Chi,
I'm sure the owners of your store would disagree. And if they wouldn't, then they're really harming their bottom lines. If it weren't for customers you (and others) wouldn't be employed.
I did retail. Currentlly I work with an automated voice response system for a large bank. Believe me when I say that I can understand that certain customers are more work than others and don't 'get' the system or the process. But I'm fully aware that w/o customers, I wouldn't have my job. The perceived fault of the customers not understanding something is not THEIR fault; it's mine. It's not their problem; it's mine. They are not supposed to be the experts, I am.
"
Waste 20 seconds on the customer" -- WOW, that speaks volumes.
I'm curious, have you said this to the owner?
This is why people are moving towards the internet for their shopping.