How to say "thanks for the trouble" in Chinese

This is a fixed expression: 麻烦 (máfan, 'trouble') + 你 (nǐ, 'you') + 了 (le, particle indicating completion). It literally means 'I've troubled you'.

Say this after someone has helped you, like giving directions or doing a favor. It's polite and shows appreciation. Don't use it for simple transactions like buying something; it's for when someone has put in extra effort.