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At SXSW 2012, online coupon website RetailMeNot.com has partnered with CardSpring to let festival-goers tie coupons and discounts directly to their credit and debit cards. The promo was executed via a campaign called "What the Truck?" in which RetailMeNot utilized the CardSpring API, a developer platform which enables the building of mobile and web applications for payment cards.

It's not as confusing as it sounds. In What the Truck's case, the RetailMeNot program used CardSpring's tech to build out a new way to link coupons directly to people's plastic cards—whether debit or credit—which frees users from the inconvenience of having to print or bring along those extra physical coupons. All they have to do is get the web app, which lets them browse vouchers for food trucks around the vicinity of the SXSW conference. Then, users can sign up, register their credit or debit cards, link the coupon, and pay normally with their card at the truck. The linked coupon is automatically applied at the time of payment, letting the user effortlessly shave $2 off a delicious Texan taco.

And that's just one application for CardSpring. "CardSpring is a platform—the way iOS is—that lets developers create programs to bridge the physical act of paying with the online space," explained Jeff Winner, CTO of CardSpring.

Winner gave us a few other examples of how CardSpring's API could be utilized. Someone who buys a drink at Starbucks, for instance, might be able to unlock a corresponding virtual badge on Farmville. Or someone might build a "verified Yelp," where users would only be allowed to write reviews for a place if they had purchased something from them earlier, information which an app using the CardSpring API would be able to verify.

Winner revealed that there has already been a lot of interest from the developer community over the API, but also that it's somewhat difficult to nail down how huge apps built for this platform could get.

"It's like when Apple came out with iOS. Some apps grew incredibly popular, while others didn't fare so well. It's bit too early to tell," Winner said. "But we would like to do that—become the iOS for the mobile payment space. Then it'll be exciting to see how things will play out."