Stillness

[aeolianchaos]

Why is it their responsibility?

She is having her mail forwarded to their house until she gets back.

Leave it where it is. She can deal with it when she gets back.
Not your problem.

I could do this, but in a way I feel like I'd be engaging in passive aggressive behavior.

Also, she has been pretty good about dropping off any bills that accidentally arrived at her place - the post office decided we had BOTH moved because one of us moved.