To close the deal, the seller threw in a one-year warranty for free.

threw in 是动词短语 throw in 的过去式。throw 本意是"扔、投掷"，in 表示"进入"。 throw in 作为短语动词，意思是"额外赠送、免费添加"，通常指在交易中主动提供额外的好处或赠品。