The window clerks are a small percentage of the postal workforce. Except for mail delivery, most of the grunt work of the USPS occurs behind the scenes between dusk and dawn and I can assure you from first hand experience that many of those people work very hard and conscientiously.
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The postal grunts, particularly the perdiems are the most abused, poorly used, overworked workforce in the States.
It’s not a good job to try to grow old in. Those parcels and sacks of mail do not sprout wings and move themselves and there’s always a deadline. Whether USPS, UPS or FedEx, people who do that work have my respect.