Sforno on Genesis
תשים בפי אמתחת הקטן, in order to test how the brothers would react to Binyamin becoming involved as the victim of a potentially life-threatening accusation, and what the brothers would do in order to save him.
Siftei Chakhamim
גביע is a tall goblet... Since it is written גביע twice, Rashi deduced that it means a tall cup. We need not ask: Why was Yoseif not concerned they would [later] claim the goblet was theirs? Because [the answer is:] It was for divination, so surely it was [not theirs but] Yoseif’s. For it says, “There is no divination amongst Yaakov” (Bamidbar 23:23). (author’s commentary)