The formula by which the postal service sets rates for parcels was set by congress in the 2006 law that required the postal service to prefund 75 years worth of pension and health care costs. Maybe that law needs to be revisited. But because this area of business is subject to competition the costs the USPS charges for these services can only go so high before Amazon/UPS/FedEx can swoop in with other business.
A lot of people here won't like this proposal, but we could always grant the USPS a monopoly on small package delivery, or whatever the technical mail term for that may be, to force more business into the USPS. This in addition to postal banking, other pilot programs to create new revenue, and reform of how the USPS funds it's pensions and health care obligations would shore up the system for time to come.