I had a friend from South America who was visiting, and we were in a clothing boutique and she wanted to ask for a discount. I was mortified, I told her we don't do that here, lol. She asked anyway, and while the store owner said no to the discount, she offered to throw in a pair of earrings. I was floored.
I wouldn't put up a sign and risk offending someone who might mean no offense in asking...