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A large part of USPS troubles are due to labor agreements unheard of in the private sector. As an example, during times of low volume, postal workers may be paid to literally do nothing: they show up, head into the break rooms, and spend the day reading or watching tv - for which they are paid their standard wage. As well, benefits are incredibly generous; labor accounts for 80% of USPS costs, compared to 32% for FedEx and 53% for UPS.
If only AlGore hadn't helped to invent the Internet, USPS wouldn't be in this pickle!