(Shannon Stapleton/Reuters)

The U.S. Postal Service mail truck, with its navy blue eagle’s head and beak on the side, is a reminder that the postal carrier still delivers the mail every day, even if Americans don’t write too many letters anymore.

And now, it turns out, one of the largest vehicle fleets in the country has reached the end of its useful life. The question is what will come next.

The answer isn’t as easy as it might seem. The delivery and collection fleet of 190,000 postal trucks includes 142,000 vehicles that desperately need to enter retirement, with an average age of 24 years and some as advanced as 27. The first Grumman Long Life Vehicle, as they’re called, rolled off the assembly line in 1987.

Maintenance costs on the trucks increase every year–and hit almost $1 billion in 2012 –and replacement parts for older models are scarce. The trucks are not up to today’s safety codes. They get terrible gas mileage. The USPS Inspector General’s office said in an audit this summer that the agency can keep them safely in use only until fiscal 2017.

But replacing the fleet would cost postal officials about $5 billion, and that’s a check that at the moment they can’t write. The financially struggling mail agency has exceeded its legal borrowing limit and doesn’t have the cash for such an expensive, if critical, investment.

The Postal Service has come up with a possible solution: Find a company that can retrofit the existing light-duty trucks by keeping their aluminum bodies and replacing the frames.

“The bodies are still in really good shape,” said Sue Brennan, a USPS spokeswoman.

The agency put out a Request for Information this week to vendors with ideas for building a new rolling chassis for the trucks that would keep delivery features like roll-up rear doors, sliding side doors and right-hand-drives for curbside deliveries — but makes other crucial updates.

Among the biggest changes, Brennan said, is likely to be increased room for packages, the Postal Service’s biggest growth area right now and its hope for the future. A mix of truck sizes is also under consideration, with some designed for letters, some bigger ones for packages. But the fleet will need to include trucks that are agile enough to maneuver through city streets.

Postal officials have sent surveys to thousands of letter carriers to ask them what they need and want in the truck of the future About 60 percent have responded so far. At the top of the list is more room for packages. Other suggestions include easier access to the back of the truck, adjustable steering wheels and shoulder seat belts, seat belts that are bright orange and safety sensors that let carriers know if they’re about to run  something over or are too close to another vehicle.

(Related: The Postal Service is losing millions a year to help you buy cheap stuff from China)