[markem]

Why is this a moral dilemma? Either you are ethical or you aren't.

IMO, you should be on the phone with the vendor explaining what happened and letting them decide what to do. They may decide that it isn't worth having it returned and merely thank you for the effort; ask you to return it at your cost; send you a shipping label; or something else.

At the end of the day, it IS NOT your package. Steal or don't, your call.