Is it right to say "How much do I owe you"?
Solution 1:
The use is correct and yes is becoming a little old fashioned due to disuse by the general public. However, people laugh at may different things...in this case I agree with one person here that could be laughing at something else but culturally it might be something from their home country. I think its only fair to have asked why laughing because it could only be sharing in on a good joke you missed because of focusing on make your purchase. i.e. Ive experienced Pacific Asian's in job interviews to smile when they feel uncomfortable.