SPOILERS of a sort, so keep that in mind
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I am guessing that Dwight's half-brother dropped what was left of Dwight and tossed it into the mail drop (ie the map of Virginia and everything else that was in the glove compartment)? If so, I would assume that meant it was over?
I just saw it as the irony that the mail was finally delivered bringing his post card that he had sent to his sister at the start. Tragedy moves faster than the USPS I guess.
A postal worker was just delivering the mail, including the postcard Dwight had sent at the beginning of the movie.
As for whether Dwight's mission is over, I'm gonna guess "yes", as I don't see his half brother tracking down his sister and killing her.