I didn't realize this was such a problem for our postal workers.
I suppose all that walking would loosen things up a bit.
maybe they could take a cue from those Jim Dandy 'toilet-on-the-door' paintings and install a real, functional toilet inside the vehicle (on the right side of course).
since most of these trucks are stored down at the USPS annex down on Leary, they could just dump the 'special delivery' where all those other guys parked down there do.
(note: this sort of stunt would've been easier when they drove those tough, military-looking jeeps, instead of the soccer-mom mini-vans they cruise these days)