U.S. Postal Service letter carrier Juan Padilla arranges mail in his truck while on his delivery route in San Francisco, Calif. Bowing to pressure from unhappy customers and members of Congress, the U.S. Postal service will not close some 3,600 rural post offices. - 

God may have rested on the seventh day, but the Postal Service won't. Amazon is teaming up with the U.S. Postal Service to deliver packages to customers in Los Angeles and New York on Sundays.

"Here’s a dirty secret — the post office has been delivering on Sundays for quite awhile for its Express Mail Service," says Carl Howe, Vice President of Research at the Yankee Group. "So this is an opportunity to take advantage of infrastructure they already have in place and make some more money."

The post office brings in about the same amount of money as Amazon does every year: Roughly $60 billion. The difference is the post office’s business has been steadily declining.

"This comes at a very good time because they’re starved for cash and this will hit the bottom line immediately," says John Callan, Managing Director with Ursa Major Associates and founder of Postal Vision 20/20--an independent entity which studies the postal business model. Callan says teaming up with a progressive online company shows that the Post Office is starting to look to the future. "It helps them play into the growing marketplace of e-commerce so that they don’t have to worry so much about the dying marketplace of letter mail."

For Amazon, this is all about customer service.

"It’s about time that they did something like this for guys like me," says Tim Bajarin, a tech consultant with Creative Strategies. He's a member of Amazon’s prime service, which offers free 2 day shipping. He says it’s frustrating to order something on a Thursday and know it won’t arrive until the following week. "If you’re eBay, Amazon or Google, you still have to deal with instant gratification of a person going to a store and buying what he wants now,"

Bajarin says this deal means Amazon can get the customer closer to the immediacy of the brick and mortar experience without them having to schlep to the store.

Follow Stacey Vanek Smith at @svaneksmith