When customers asked if it would kill me to smile, I would give them the fakest, deadest smile on the planet for about a half a second.
If I felt like smiling, I would. My motto is, I don’t get paid to smile like a person on Prozac. I get paid to help you, and take your money.
As for customers who have, um… unusual tactics to get attention, when I worked the electronics department I would get waved at from across the department, which gave me a case of intermittent astigmatism.
I’d get yelled at from the same distance: “HEY!”
Which of course they were yelling at their kid, wife, shopping partner, etc. Ignored.
And on occasion I’d get whistled at, which caused me to look around to see if a dog was loose in the store. None of these tactics actually got me to pay any attention to you.
And if, when you actually said "Excuse me," and then asked why I was ignoring you before, my polite reply would be, "Oh, I’m sorry, I thought you were waving to/yelling at your wife/son/friend. How can I help you?"