The USPS station in Irvine is the first in the United States to test drive a Chevrolet Equinox as part of General Motors' Project Driveway, which puts more than 100 fuel-cell vehicles in the hands of average drivers as part of a large-scale market test.
A second station on the East Coast will also receive an Equinox. The city has not been announced.
The hydrogen-fuelled vehicles are considered more environmentally friendly because they emit only water vapor.
By participating in the demonstration programme, the postal service will explore the car's ability to handle the demands of a delivery vehicle, which makes multiple stops daily and is used six days a week.
"There's a lot of learning that we can get out of these vehicles that will help the nation," said Walter O'Tormey, vice-president of engineering for the postal service.
The US Post Office operates the largest civilian fleet of vehicles in the world and O'Tormey said it was spending an extra US$600 million ($818.2 million) in gasoline this year.
He noted that a US1c rise in the cost of a gallon (3.78 litres) of fuel adds US$8 million to its annual expenses, which were US$1.7 billion last year.
The postal service relies on the sale of postage, products and services, not tax dollars, to pay for the cost.
"That's why we're looking for a vehicle that reduces, or eliminates our dependence on petroleum products, that will create a healthier environment for the next generation."
He said letter carriers were testing several different technologies in 43,000 vehicles.
They include vehicles that run on ethanol, electricity and natural gas.
USPS staff will fuel the vehicle at a hydrogen fuelling station at the University of California, Irvine, campus. GM will provide the maintenance, fuel and service of the vehicle.