chakeretam is very informal, I would say it is even used by low-class people or low educated people.
mamnoon is quite correct and normal.
You can use these expressions too:
moteshakkeram (mochakeram) (formal)
lotf kardid (normal)
sepasgozaram (very formal)
Hi, Lilia. Both Google and the native speaker are right, as both words, spelled correctly here, are expressions of gratitude.
‘Thanks’, as in ‘she has received no thanks for her services’, stands for تشکر, with the difference that the Persian word is actually singular rather than plural. It is simply a curt way of saying ‘thanks’, just like so.
متشکر means grateful and is, again, a shortened expression for متشکرم, motshakeram, ‘I am grateful’.