I'm going to guess that the net closing speed was not all that high. The USPS vehicle would have been going relatively slowly and you were at more or less a right angle, so not the problems you would have in a head on at 60 MPH each. My best guess is that while your exit port was sucking the seat in, you pounded the brake pedal with vigor. I sure have.
While this sucks, it could have been worse and your limited injury is a blessing. I have to concur with the concerns about the USPS driver; that is a hard error to make under what looks like good visibility. I would probably have been cranky and disagreeable.