Amazon is testing its own shipping service, a move that would streamline the online retailer's delivery process but could hurt major shipping services like FedEx and the U.S. Postal Service.

The company has been developing delivery networks that can take packages the "last mile" to the customer's house, testing in San Francisco, Los Angeles and New York, The Wall Street Journal reported.

Amazon began rolling out the shipping service late last year, labeling packages with "AMZL" and "AMZN_US," codes that signify the internal delivery service, people familiar with the matter told WSJ.

If Amazon decides to implement its own shipping service, the company will have new control over consumer experiences and curb shipping costs. But the shift wouldn't be so helpful for FedEx, UPS and the U.S. Postal Service.

The company said in a recent job posting, "Amazon is growing at a faster speed than UPS and FedEx, who are responsible for shipping the majority of our packages.

"At this rate Amazon cannot continue to rely solely on the solutions provided through traditional logistics providers," the posting said, according to WSJ. "To do so will limit our growth, increase costs and impede innovation in delivery capabilities."

Besides changing up the game for FedEx and UPS, the shift to an internal delivery service would also further establish Amazon as a competitor with Google, eBay and Wal-Mart, all of which are testing their own delivery infrastructures.

Amazon has already been testing deliveries in the U.K.

"We've created our own fast, last-mile delivery networks in the U.K., where commercial carriers couldn't support our peak volumes," CEO Jeff Bezos said in his annual letter to shareholders earlier this month, as quoted by WSJ. "There is more invention to come."

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