By JP Mangalindan
February 3, 2014

FORTUNE — How much more would customers pay for an Amazon (AMZN) Prime membership?

That’s the question Amazon is mulling over. During Thursday’s quarterly earnings call, CFO Tom Szkutak announced that management might up Prime’s $79 annual fee by anywhere between $20 and $40 due to increasing fuel and shipping costs.

The news surprised analysts. “The way they went about announcing it just didn’t seem very Amazonian,” says Mark Mahaney, managing director at RBC Capital, who expected a digital letter penned by Jeff Bezos on, if anything. More importantly, Mahaney says he doesn’t see “a financially obvious reason” to raise prices, pointing out that Amazon’s shipping costs seemingly remain stable.

If a Prime price hike is in the works, the move could affect Prime’s substantial U.S. user base, which could range between 10-15 million according to RBC Capital, depending on how it is executed. Raising the standard price as high as $119 a year — a 50% increase — would be asking a lot of members, spurring some to end their membership.

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Instead, Mahaney predicts Amazon may take a different route and introduce a tiered pricing system: the higher the annual the fee, the faster the shipping. For $99, members could expect guaranteed next-day, sometimes same-day, shipping. Meanwhile, users could expect standardized two-day delivery for $79.

Since its introduction nine years ago, Amazon Prime has evolved into an amalgamation of the company’s different services, including shipping, e-book lending, and video streaming. Last year alone, management disclosed the number of TV and movie streaming available had climbed from 33,000 to 40,000 movies and TV episodes.

Not only have those myriad features made Prime a more appealing proposition for customers, Prime members are emboldened to spend more because of expedited shipping. Consumer Intelligence Research Partners (CIRP), a Chicago-based equities securities research firm, estimates Prime members spend roughly $1,340 a year, compared to just $708 a year for non-Prime customers. (Mahaney goes further, calculating Prime members can spend up to four times their non-Prime counterparts.)

In the end, higher prices needn’t mean bad news for Amazon if it plays its cards right. Says Mahaney: “This could actually be very positive for Amazon’s fundamentals.”

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