I'm thinking of throwing out a box of things that belonged to my boyfriend's former girlfriend because she probably doesn't need the stuff any longer.
What makes you think the box belongs to her?
It says so on the box.
Well, then you know that this box doesn't belong to you, and that your boyfriend, at one point, made a decision to keep it. Regardless of how you feel about the situation, it is not up to you to decide what to do with his possessions. Maybe you could convince him to return those items to his ex.
I guess I could do that instead. Perhaps there are things in there that she may want to have.
Sounds like a good plan. I'm sure she'd appreciate that.