Now the letter carrier will deliver groceries.

Amazon.com Inc. and the U.S. Postal Service, already partners in Sunday deliveries, have launched a trial to shuttle insulated containers of meat, dairy, produce, and other groceries to San Francisco customers’ doorsteps. The test could presage a broader national rollout, giving Amazon’s grocery drop-off service a much wider reach.

Through its AmazonFresh unit, the e-commerce giant today delivers groceries in its hometown Seattle, as well as Los Angeles and San Francisco. It is expected to introduce Fresh to additional markets in the coming months: Amazon AMZN, -1.78% lists job openings for produce-handlers in Avenel, N.J., about 25 miles outside Manhattan.

If successful, the two-month test could provide a boost to the Postal Service, which has aggressively chased a bigger piece of the e-commerce pie as the Internet chips away at other parts of its business. Its package business is up 20% over the past five years to 3.7 billion packages.

The Postal Service said it is testing AmazonFresh deliveries “to determine if delivering groceries to residential and business addresses would be feasible from an operations standpoint and could be financially beneficial for the organization.”

An expanded version of this report appears at WSJ.com.