SEATTLE – As the holiday shopping season revs up, is adding another weapon to its competitive arsenal: Sunday delivery.

The online retail giant has cut a deal with the U.S. Postal Service to extend delivery of packages to customers to every day of the week. That means Amazon Prime customers who pay $79 a year to get free two-day shipping on millions of products can order an item Friday and have it arrive before the weekend is over. And Sunday delivery will be available to customers who don’t subscribe to Prime, as well.

Sunday deliveries of Amazon products will start this weekend, but only to customers in the Los Angeles and New York metropolitan areas. The company, which needs to build up the technical capability at warehouses elsewhere, plans to work with the Postal Service to roll out the Sunday delivery to “a large portion of the U.S. population” next year.

Details of the agreement have been sealed with the Postal Regulatory Commission, an independent agency with regulatory oversight over the Postal Service, according to USPS spokeswoman Sue Brennan.

It’s the first time the Postal Service has worked with a retailer to enable Sunday delivery.

“It requires the scale that Amazon has,” said Dave Clark, Amazon’s vice president of worldwide operations and customer service.

On Nov. 26 last year, Amazon’s peak shopping day, customers ordered more than 26.5 million items worldwide, or 306 items per second. About 57 percent of Amazon’s sales last year were in North America.

For the Postal Service, the deal boosts volume, something the financially struggling agency needs. It briefly flirted with canceling Saturday mail delivery earlier this year, only to have Congress pass a short-term budget that required six days of mail delivery.

Despite the diminishing economics of mail delivery, package delivery is booming for the Postal Service. Agency spokeswoman Brennan said the Postal Service expects to deliver 420 million packages in the period between Thanksgiving and Christmas, up by 12 percent from last year.

During the holiday season, that volume often leads to massive package backups on Sundays.

“Mondays are historically huge delivery days,” Brennan said. Part of the deal with Amazon, she added, “is to ease the weight for our carriers.”

For Amazon, delivery speed is a crucial competitive challenge. It can often beat brick-and-mortar stores with prices. But it can’t match consumers’ instant gratification in obtaining a purchased item the second it’s bought.

What’s more, Sunday delivery ratchets up pressure on Amazon’s online rivals, which haven’t cut Sunday delivery deals with the Postal Service. Amazon’s deal is not exclusive, though Brennan said the agency is not currently negotiating with any other company.