The U.S. mail truck is something of a cultural institution, so the U.S. Postal Service’s job of replacing it with a modern alternative is a tough challenge. Now, the agency says it will cost an extra $3 billion to make those new mail trucks run on electricity.

As , the USPS has published its final Environmental Impact Statement for the new vehicles. In the report, the agency outlines the ambitions, limitations and costs of a next-generation mail truck.

The new mail truck will replace the USPS’s aging fleet of 212,000 active vehicles across the country. Specifically, the agency is looking to upgrade its purpose-built, right-hand-drive mail trucks, which were designed for curbside deliveries.

These trucks, the USPS said, have “far exceeded their planned life.” The expected service life of these trucks was 24 years, and the average age of the vehicles still out on the road is now 30 years. Time for an upgrade, it seems.

The USPS said it has two options for replacing these trucks. Either a new, more energy-efficient internal combustion engine (ICE) truck, or a battery electric vehicle (BEV).

And it’s this BEV that sounds like an interesting proposition for the postal service.

on a single charge. This, it said, would be sufficient to cover around 95 percent of its routes.

The truck should also be capable of carrying a payload of 2,207 lbs and pack in a 94 kilowatt hour battery, which can be fully charged overnight. If a truck that meets these demands can be built, .

And when it comes to the environmental impact of the new trucks, the USPS said even a switch to 10 percent BEVs could dramatically cut its emissions. According to the report, the USPS could reduce its CO2 emissions by 256,850 metric tons each year if it switched to battery-powered mail trucks.

But, there’s a catch.

In the report, would come with significant additional costs.

A full BEV fleet would “require over $1 billion more in additional investment” the report warned. This would cover the cost of the vehicles, training, manuals, charging infrastructure and 20 years-worth of fuel and utility costs.

As such, a fully electric postal fleet would cost $11.6 billion over 20 years. In contrast, a fleet comprising 10 percent electric trucks and 90 percent newer ICE models would cost $9.3 billion over the next 20 years.

As well as the additional costs, the USPS warned that “operational limitations and certain Postal Service delivery environments would currently limit the use of electric-only vehicles.”

But, rather cleverly, the USPS has thought of a solution to the current limitations of BEVs. The USPS said that any next-gen mail truck should be capable of being retrofitted to run on battery power.

Meaning that while it pledged to initially purchase at least 10 percent of its new fleet as BEVs, that number could increase as the technology advances in the coming years.

So, with the USPS anticipating , it’s an exciting time to be a mail carrier.