Hi, bravo Simon, that's exactly what " متشکرم " means .
About the word "merci", I've gotta tell you, that is a French word, and it's like when an American says "Adios" instead of "Goodbye" , now you tell me Simon, which one is correct to use in American Language ?- adios or goodbye ?
So, if you're talking in Farsi, you SHOULD say " متشکرم " instead of " مرسی " .
موفق باشی = Good luck