Kind of a general suck I keep running into, but if you are working and need a few minutes to finish up cleaning or organizing around your counter, could you at least say, "Hi, I'll be with you in a minute?" I don't know what you are doing or how long it's going to take, but I'm perfectly happy to wait for you if you just take a second to acknowledge my existance. Especially after I say "Hi" or "Excuse me" several times. It's just good manners.
I used to do this all the time when working retail or on phones. It helped ease a lot of customer suckatude.