Thank you. "In congratulation(s)" looks okay to me. People will have a hard time understanding what "Hand flowers" is supposed to mean. "Give flowers" would be much clearer: Give flowers to people when you congratulate them on their promotions.
Oh, one more thing: isn't "in congratulations" wrong here since (according to my dictionary) the uncountable version stands for the expression and the plural form refers only to words of congratulation?