Here 为的 is like "for the purpose ..." "so that" "in order that". The sentence does not stand alone, unless you say something like 我们提供这么多优惠条件，为的就是长期合作而不是“一锤子买卖”。 Note that, for better wording, I crossed out three characters (和你们) from your sentence, as 一锤子买卖 is a noun phrase, which does not stand as a good match with verb phrase (和你们长期合作)。 If you want to retain 和你们，you can add "做/搞" before 一锤子买卖 to make it a verb phrase.