Last year I was in Germany in Ruhr area for education as an Erasmus (European student replacement program). I wanted to buy some vegetables there, for that I went into a small shop, after a short communication cashier said to me “Lassen Sie sich!” I think this phrase means “please go ahead, you can buy whatever you want” But now I want to me make sure if is it used for a specific situation?
“Lassen Sie sich …” mostly means that you are served by someone. You are meant to stay passive and enjoy (more or less) what is done to you.
Some further examples:
Lassen Sie sich verwöhnen. — Give yourself a treat. (Precisely means “We will give you a treat.”)
Lassen Sie sich einkleiden. — I/we will dress you.
Lassen Sie sich gehen. — Just relax.
But there is also another side: “Lassen Sie sich nicht …” which means that you should defend yourself against something.
Lassen Sie sich nicht fertig machen. — Don’t let them stoush you.
Lassen Sie sich nicht abzocken. — Don’t let them rip you off (your money).