The financial services company is forging an official partnership with eBay-acquired RedLaser to offer consumers on-the-fly comparison shopping options, and to boost buying through its online rewards store, shopAmex.

RedLaser is a leading mobile bar code application, with an estimated 15 million downloads across all mobile operating systems.

All AmEx cardholders with membership rewards — whether Green, Platinum or Centurion Black Cards — that download the free app to their smartphones will be able to scan item bar codes, find out how many AmEx rewards points they’ve accumulated and potentially purchase the same item at a discount on shopAmex, a catalog of five million products from approximately 60 online retailers like Target, Saks and Apple. About four million of the listed products will be available to cardholders that use the RedLaser bar code scanner.

For American Express, the RedLaser deal is part of a growing push toward serving customers through mobile. The company’s most recent earnings report shows that AmEx has 95.8 million cards in force, including consumer, corporate and small-business cards; and according to the company, an estimated two-thirds of cardholders with membership rewards use a mobile or tablet device.

“In everything we do right now in terms of mobile and tablet, we’re trying to figure out ways to deliver value to our customers, because this is where it is right now: They’re using their phones to shop,” said Dustin Harris, director of interactive development at American Express.

For retailers — and consumers — it goes to show that it’s a brave new comparative world out there, with more shoppers having access to potentially better price points right in the palms of their hands, via smartphones. As my colleague Tricia Duryee reported recently, mobile shopping shot up during this year’s Cyber Monday record-setting sales, and mobile is expected to spur even more buying as smartphone adoption increases.

The RedLaser app for AmEx membership rewards will work only on the iPhone to start, though the company expects an optimized iPad app to become available soon.

Update: A previous version of this article suggested AmEx users could use the RedLaser bar code scanner to cash in on membership rewards and buy an Apple iPad. Cardholders will not be able to scan a bar code to buy an iPad through shopAmex.

A savvy move for Amex, too. Though Revolution competes against entrenched credit-card companies and PayPal, among others, its alternative payment system, which reduces costs for accepting credit cards by up to 75 percent, is quite attractive to merchants who’ve shouldered those costs for so long.

Says JP Morgan analyst Imran Khan: “While it is hard to know precisely what direction AmEx plans to take the Revolution assets, we think the acquisition suggests the company is trying to be more aggressive in the online payments arena. We think PayPal’s existing account base and international footprint have given it a network advantage in the C2C space that is hard to dislodge. On the other hand, we believe significant room exists for market share gains in Online Payments by companies that offer innovative solutions, and this acquisition gives Revolution Money a stronger backer.”

You’d have a market that barely exists yet is worth fighting over. At least that’s what Zong, a Swiss-based company, and Boku, a rollup of two other mobile payment companies (Mobillcash and Paymo) are doing. Both offer the same thing: The ability to buy stuff online that gets billed to your wireless account.

In theory, you could use the same technology to buy actual stuff as well, but the businesses are really geared around microtransactions–pretend weapons on the “Mob Wars” Facebook app, piratey stuff on Puzzle Pirates–for which it wouldn’t make sense to use a credit card. They’re also assuming that many of their customers won’t have credit cards, either because they’re kids or because they live in countries where it’s more common to own a mobile phone than a Mastercard.

Both companies are happy to explain why they’re better than their rivals–who covers more territory, who has the better carrier relationships, who has less onerous fees, etc.–but I won’t bore you with that. Similarly, while Boku is formally announcing its presence today, along with $13 million in funding from Benchmark Capital, Index Ventures and Khosla Ventures, the Zong guys argue that this is really a rebranding of Mobillcash, which had previously received funding from Index and Khosla. Whatever.

The real story here will be if and when the really big platforms for virtual goods–like, say, Facebook, News Corp.’s (NWS) MySpace and game companies like Electronic Arts (ERTS)–decide they want to formally integrate one or more of the mobile payment guys, and under what terms.

Right now, for instance, Facebook isn’t involved in any of the microtransactions that any of its apps are generating, and the same goes for mobile payments–each app is free to work with whatever vendor it chooses. But that could change in the near future as Mark Zuckerberg and company start testing their own payments platform. If there is going to be a big market for virtual goods bought via mobile payments, it could get carved up in the near future.