Square, the company that makes those little headphone jack credit card readers, would like you to use their services more. Of course they would - that's kind of the whole point of commerce. But at the moment merchants using Square are limited to the dedicated Square app, which by default is a sort of digital cash register. The company is hoping to expand itself a bit with its latest API, which allows developers to integrate Square payments into their independent Android apps.
The system isn't quite as streamlined as it might otherwise be - users will still need the Square app installed on their phone or tablet, and once the API is activated it pops out into the dedicated Square app. But the API should allow more apps to take advantage of Square's ultra-mobile solution for easy retail payments, and that's a good thing if you're trying to sell something without a lot of expensive infrastructure.
The API only needs three lines of code to initiate the hand-off, then the payment results are returned to the original app once Square is finished. Unfortunately the API is only available to developers in the US and Canada at the moment, according to Square's blog post.