For decades, this is how the mail was delivered in the United States. The "vehicle" has changed over time.

The Pony Express is long gone. So are the Ford Pintos. Over the years, the U.S. Post Office has tried a lot of different ways to make its appointed rounds. These days, carriers generally settle for a mix of minivans and aging Grumman trucks. But in its bid to become more efficient – and competitive with package services like UPS and FedEx, the USPS is looking to find its “next-generation delivery vehicle.”

It’s no small task. The post office is eventually expected to need 180,000 new trucks, so the eventual contract is likely to be worth something on the order of $6 billion.

“Though the existing fleet has served the Postal Service well, it has become expensive to continue to maintain the aging vehicles,” said USPS spokeswoman Sarah Ninivaggi.

Most of the Grumman trucks now in the post office fleet are at least 21 years old. So, maintenance is a big issue. But so is the cost of fuel, the money-losing service spending $539.7 million to gas up its trucks in fiscal 2014. Transportation costs made up a sizable chunk of last year’s $5.51 billion deficit.

The US Post Office has used a variety of delivery vehicle and the current fleet of Grumman-made trucks may be replaced.

“More importantly though, shifts in consumer trends are driving a lot of factors being considered in a next-generation vehicle,” said Ninivaggi.

The USPS has been struggling to balance its books, in part, due to the decline in first class mail volume as more and more communications shifts to the Internet. At the same time, the post office was slow to catch onto another Web-led transition: online shopping. It is now hoping to capture a bigger share of the massive growth in online retailing largely dominated by FedEx and United Parcel Service.

The bottom line for the Postal Service? It needs bigger, more flexible trucks that are also more fuel-efficient.

Expect to see some sort of van-like configuration, the initial specifications calling for a “fully enclosed van style body.” The new trucks will need to be big enough to carry a passel of parcels, everything from CDs and books to the beds and other big items that Americans routinely order online these days.

Drivers will need to be able to stand up and walk inside the new trucks, something they can’t with either the minivans or Grumman models currently in use. There will need to be shelves to better organize things, as well, the post office has told automakers.

The USPS is looking to spend somewhere between $25,000 and $35,000 per vehicle.

The timing may be particularly good for the USPS. After years of design stagnation, automakers have been flooding the market with an assortment of new commercial vehicles. Ford recently updated its compact Transit Connect van while replacing its aged Econoline with the full-size Transit model. One version is tall enough for a delivery person to easily walk through.

Could this be the next delivery vehicle for the USPS? The Sprinter van offers a lot of flexibility that the current post office needs.

Daimler last week announced it was building a new plant near Charleston, South Carolina, to assemble its big Sprinter van. In the past, it shipped partially assembled “knock-down” kits to a smaller South Carolina facility for final assembly. The German maker also is launching a smaller van, the Metris, to compete with the likes of the Ford Transit Connect.

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Nissan and Fiat Chrysler also are pushing new delivery-style vans. The Japanese maker, meanwhile, recently began rolling out converted versions of its NV van which will become New York City’s new “Taxi of Tomorrow.”

Whether one of these might fit the bill is uncertain. In fact, non-traditional competitors might want to enter the bidding – as Grumman did decades ago – or one of the automakers might come up with a distinct USPS model considering the side of the potential deal. (For comparison, Daimler sold barely 26,000 Mercedes-Benz Sprinter vans in the U.S. last year.)

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The post office is optimistic it can justify the big order. While still lagging against the private shippers, the Postal Service handled 524 million packages in December, the year’s shipping peak, an 18% year-over-year increase.

Along with more space and easier access, the new trucks will be expected to be cleaner and a lot more fuel efficient than the Grumman trucks which barely get 10 miles per gallon. Among other options, the USPS will consider alternatives such as hybrids, CNG, even fully electric powertrains.

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The quasi-government agency met with auto industry officials last month to give a preliminary overview of its plans. It has yet to lay out a specific timetable, though it’s expected to take several years to get the entire Postal Service fleet upgraded. Considering the timeline for the typical bidding process and the possible need for major changes to existing truck designs, that could push the project out to the end of the decade or beyond.