Just how big are Amazon's (AMZN) - Get Report ambitions in shipping?

A recent earnings report from XPO Logistics Inc. (XPO) - Get Report , a major provider or warehousing and last-mile delivery services, provided some clues. 

Last week, XPO executives told investors that it missed quarterly revenue forecasts due in part to a pullback from a major customer, widely assumed to be Amazon, which was equivalent to two-thirds of that particular revenue stream. XPO is also closing a number of facilities nationwide as a result of the pullback. 

Amazon was believed to be XPO's single biggest customer, and the repercussions were immediate for its logistics provider, with shares down 9% since XPO executives revealed the news last Friday. But the XPO report also spotlights Amazon's increasingly aggressive stance in the transportation and logistics arena. (XPO closed down about 3.5% on Friday to close at $52.17.)

In its recent 10-K filing, Amazon also suggested that businesses it once regarded as partners are now competitors, listing "companies that provide fulfillment and logistics services" in the competition section of the filing.

Apart from the XPO drawback, Amazon has been making other moves to bring the transportation and delivery logistics side of its business in-house instead of relying on third parties. It's opened an Amazon Air Hub in Kentucky, for example, and is also expanding its home delivery and trans-Pacific shipping operations according to recent reports.

On its fourth quarter earnings call, Amazon CFO Brian Olsavsky told investors that handling more transportation on its own is cost-effective: "What we like about our ability to participate in transportation is that a lot of times we can do it at the same costs or better, and we like the cost profile of it," he said.

The question is how far Amazon plans to go in building out its transportation and logistics capabilities. 

In a January note, Loop Capital's Anthony Chukumba, wrote that if Amazon aims to seriously build out its transportation capabilities, buying a provider such as FedEx (FDX) - Get Report is a better option than building a comparable network in-house. "Amazon could make an accretive acquisition of the best global network for a fraction of the cost of building it themselves (we recently attended a meeting at FedEx headquarters at which the CFO made exactly the same point)," he wrote, also noting that if Amazon were to become a serious threat to a FedEx or UPS, it could also lead to aggressive price hikes by the incumbents.

There are other potential risks to Amazon running its own transportation and logistics network, noted Nick Vyas, executive director at the Center for Global Supply Chain Management and professor at USC's Marshall School of Business.

One reason is that for a company as large as Amazon, becoming an end-to-end logistics provider -- on top of its other dominant services in ecommerce and AWS -- could draw harsher antitrust scrutiny, which Amazon has managed to avoid thus far.

"Because [Amazon] was a startup, and the industry was brand new, policymakers were so unaware of the scale of the growth in ecommerce that they've ignored this space," he said.

Another is that logistics businesses have different overhead structures than what Amazon is used to, making an acquisition of a player like FedEx or XPO potentially risky. Other than the FedEx buyout idea, Amazon was once reportedly in a bidding war for XPO with Home Depot (HD) - Get Report .

"FedEx, UPS (UPS) - Get Report , freight companies also come with large overhead and challenges -- the industry is controlled by pilots or mechanics, and suddenly you have a business unit with a labor union engaged. Are you creating the risk profile of being completely unionized? That would be a completely different Amazon," Vyas added.

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