Not that it matters to those of us who are generally happy with the value & quality of USPS service ... nor does it matter that like any large organization there are screw off employees ....
The enabling legislation requires USPS to fully fund future health care & retirement for employees / a good move so that it does not build up big future unfunded liabilities. So some years it has a surplus of current revenue over current expenses / some years a deficit. The "loans" of which you speak are a cash flow issue. Again, there is NO direct taxpayer funding of USPS. Also, USPS has for decades used many private contractors to move mail between BMC - star route contractors, airlines, railroads, UPS, Fedex etc. This is not a complaint - an observation about operational efficiencies - a good thing to keep mailing rates from being higher than they would otherwise be.