Definition of “throw sth in” - Learner’s Dictionary

throw sth in

phrasal verb with throw uk us /θrəʊ/ verb [ T ] (past tense threw, past participle thrown)

to add something extra when you sell something and not increase the price:

They're selling computers with a free printer thrown in.

(Definition of “throw sth in” from the Cambridge Learner's Dictionary © Cambridge University Press)