"Throw in" is a phrasal verb that means to add something extra or make a concession. When written down, it can be replaced with terms such as "include", "add", "offer", "contribute", or "donate". Synonyms for this phrase can also be used depending on the context in which it is used, such as "put in", "mix in", "factor in", or "put forward". Each of these words can be used to convey a slightly nuanced meaning, but ultimately, they all suggest some form of addition or compromise. Being familiar with these synonyms can help in adding variety and clarity to your writing or conversations.