So a few things worth mentioning.

Bulk junk mail is the only thing still keeping the USPS alive. If it weren't for the junk mail they wouldn't make enough to keep the lights on let alone pay their people, pay for their buildings and etc.

If you go to Fed Ex or UPS and try to ship a letter it's a $3 minimum charge. Many people believe it is that expensive because shipping letters is very costly and cheap shipping is not profitable which is a leading cause of USPS current woes. That is not the case at all. UPS and Fed Ex could likely ship a letter for 10 cents or so but it is illegal for them to do so. Thy are required under federal law to charge (I believe) $2.40 or $2.50. The USPS knew that privatized couriers would put them out of business so they had a law passed to make sure they were the only organization in the letter game.

How did the decline happen so quickly? Electronic funds transfers have taken over. The younger generation checks their mail once maybe twice a week. Everything is now done paperless and all the bills are paid online. The transition from paper letters and mailing checks happened in less than a decade and the decline will continue until there is nothing left but junk mail which is likely only a couple years away. Hell even writing letters to prison inmates and sending them money for their books is now done online. Add in the fact that the USPS has MASSIVE legacy costs due to pensions and lifetime health benefits to retirees and you get the perfect recipe for financial disaster.