Symbolism, Imagery, Allegory
While all the items in the breakup box symbolize some experience shared by Ed and Min, some of them have special import. Chief among these symbols is a set of keys, which are originally part of Min's Halloween costume. Ed and Min have a couple's costume of sorts (he's a prisoner and she's the warden), and Min plans to give him the keys at the end of the night. She doesn't mean anything in particular by this gesture (at least not consciously), but when Min tells her mom about her intention to hand over the keys, Min's mom sees the writing on the wall:
My mom's eyes widened.
"What?"
"You're going to give Ed those keys?"
"What? It's my money."
"But Min, honey," she said, and put her hand on me…. "Isn't that a little, you know?"
"What?"
"Symbolic?" (29.26 – 29.32)
Min plays dumb, but in the next chapter she's tells Ed, "I want to give you my keys" (30.5), by which she totally means her virginity. Long story short: Min's mom is totally right.