AmazonFresh, the grocery delivery service run by the retail giant in selected US west coast cities, has started a trial program which sees the US Postal Service shipping its items. The 60-day program, which started in August, tasks the USPS with delivering food items, toiletries, and other everyday essentials to customers in the San Francisco area. Items are scheduled to be delivered between 3 AM and 7 AM, when most of the USPS' trucks aren't in use anyway. AmazonFresh's perishable food items come shipped in insulated bags, meaning that the trucks don't need to outfitted with costly refrigeration units.

The US Postal Service already handles a reported 35 percent of Amazon's shipping, but the USPS been strengthening its ties with the online retail company as it focuses on the package segment of its business, which has grown 20 percent over the last five years. Last year the USPS hammered out exclusive contracts to deliver Amazon items on Sundays in more than 20 US cities.

For now, AmazonFresh delivery is only available in Seattle, Los Angeles, and San Francisco, but The Wall Street Journal says Amazon has job openings for produce-handlers in New Jersey, hinting at an expansion to New York City. Last year, Reuters said the company was even considering expanding outside the US as it aimed to introduce the grocery delivery service to 20 more cities in 2014.