Customers think they're funny.
Sometimes situations arise when I don't know the price of something immediately. (This can happen with items that have been returned from online, for instance.) Customers always feel the need to chime in, "That means it's free, right?"
No, it doesn't, and you're not funny. You're actually distracting me from figuring out the price.
Also, don't ask me for my discount. You can't have it. First of all, we're not even friends. And secondly, I can't even give friends my discount. Especially don't ask me if my discount is "100% off." It's not.
And guess what.
You're still not funny.
And after you ask me for my discount, don't say, "I mean, it doesn't hurt to ask, right?" Because it actually does. It makes me angry with you, which makes the transaction less pleasant.