Amazon's drive for global domination took on a new dimension this morning, when it was revealed that the world's largest online retailer now owns a day of the week.

In Los Angeles and New York, Amazon Prime customers can now get orders delivered on Sunday by the U.S. Postal Service. The company says it plans to roll out the Sunday option much more widely in 2014, with Dallas, Houston, New Orleans, and Phoenix among the cities next on the list.

For Amazon, Sunday delivery must feel long past due. Delivering products is Amazon's core business. Imagine Google not being able to offer search on Sunday, or Apple closing the App Store one day a week, or Walmart closing all its stores. How maddening it must be for a company bent on giving people what they want when they want it to not to be able to follow through a full one-seventh of the year.

But in a country where the laws of retail commerce have long since trumped religious custom on Sundays, shipping has remained a stubborn holdout. It's extra surprising that the organization to finally break that tradition would be a lumbering federal agency rather than a private company – especially since the Postal Service has been talking about cutting rather than expanding the number of days it delivers. Yet if any company could get mail carriers out on the streets an extra day, it's Amazon.

The Postal Service over the past few decades has been ever-increasingly squeezed. Private carriers such as FedEx and UPS undercut its parcel business, and the internet has made sending physical letters and documents obsolete. Meanwhile, postal workers say a Congressional mandate to pre-fund its pension obligations far into the future has sucked the agency dry. Regardless of the reasons, the Postal Service needs more revenue. And Amazon's aggressive push to make online retail the main way people shop makes it an ideal USPS customer.

When the Postal Service's rumblings about ending Saturday mail delivery surfaced at the beginning of the year, the agency was also in the process of testing a same-day delivery service in San Francisco. The agency said at the time that while its mail business was shrinking, its parcel business was on the rise. While the plan was to halt mail on Saturdays, package delivery would continue. The message seemed to be: If the Postal Service could start offering delivery services, such as same-day, that competitors weren't, it could advance what has been the slow, painful process of transforming itself into a viable 21st-century institution. At the same time, Amazon has been building out its network of warehouses to get closer to more Americans, in the process making same-day delivery a more viable option. If the Postal Service could crack the logistics of same-day on its end, perhaps it could even become Amazon's carrier of choice.

Sunday delivery would seem to be another way for the Postal Service to seek out competitive niches, though there's nothing to indicate other carriers couldn't start offering the same thing. The weirder aspect is the way a single for-profit company seems to have deputized a government agency to serve its particular private interest. Could Walmart now ask, for instance, for a similar Sunday option? For anyone who has waited in an interminable Post Office line only to be treated like garbage at the counter, the desire to force the Postal Service to run itself more like a business is understandable. Without incentive to do better, the experience seems likely to keep sucking.

Tabling the issue of whether fault really lies with the agency itself or with Congress, however, the Postal Service for now remains a public utility. If it takes a private company to save it, it's hard to see it as public anymore. Then again, as one of the brands apparently most admired by the public, maybe no one would mind if Amazon ran the mail. Instead of stamps, think Post Office Prime.