According to the US Postal Service Office Of Inspector General:

see full report here 

The work of U.S Postal Service Office of Inspector General’s (OIG) actuarial consultant,Hay Group, determined that the Office of Personnel Management’s (OPM) assumption that the annual health care cost inflation rate will average 7 percent annually for all future years is unreasonably high. Therefore based on OPM’s assumptions and methodology, the Postal Service’s future retiree health care liabilities will be overestimated.

If the Postal Service continues the payment schedule required by the Postal Accountability and Enhancement Act of 2006 (the Act), our calculations indicate that the Postal Service could overfund its retiree health care liability by $13.2 billion by the end of fiscal year 2016. The Postal Service could pay on average $4.0 billion less each year from FYs 2009 to 2016 to prefund its retiree health benefits and still achieve the same level of funding anticipated under OPM’s assumptions. The net present value of the interest savings from the reduced payments is $5.95 billion.