iZettle — a mobile payments startup based out of Sweden that has been referred to as the “Square of Europe” for its small piece of hardware that attaches to a smartphone or tablet — is today taking one more step to making its service more ubiquitous in the nine countries where it operates: it’s releasing an SDK that will directly integrate iZettle mobile payments into third party applications.

It’s doing so ahead of U.S. counterparts like Square and PayPal — although the latter is expected to be releasing one sometime this quarter, and has already been testing it out with at notable partners like Uber.

Tellingly, iZettle cites a car service company also as a reference customer for its SDK.

“iZettle’s payment feature has become a central aspect of our application,” Zeryab Cheema, founder & CEO of Taxi 24/7 and iView Inc, said in a statement. “Customers increasingly use our app to book their journeys and the recent addition of a payment option thanks to the iZettle SDK has really helped boost customer satisfaction and repeat business.”

The SDK — which will come out first for iOS before then being extended to Android later this year — is an expansion of the API that iZettle launched last year. Like the API and iZettle’s direct solution, charges will be between 2.75% and come down to 1.5% as transaction volumes increase (a full explanation of its pricing here).

The API solution also integrated an iZettle payment option into third party apps, but when a customer used it, clicking on the “pay” button triggered the iZettle app to open. Now the experience will remain in the original app — something that will create a better user experience and be significantly more attractive to the merchants using it.

“In our ongoing quest to democratise card payments we’re now opening our platform to developers who already have an app, or are working on the next blockbuster iOS app, and need a payment solution,” said iZettle co-founder and CEO Jacob de Geer in a statement.

While iZettle very much got its start directly targeting merchants who were too small to make accepting credit card payments financially viable (because of charges from banks and the card companies), it’s increasingly been working with others to reach those small merchants and those that are bigger — a sign of how payments processed through a mobile app and dongle are becoming ever more accepted by the mainstream.

That has involved taking a $6.6 million round of funding from Banco Santander to expand into new markets in Latin America. It’s currently active in Brazil and Mexico. So far those rollouts have been going “very well,” Johan Bendz, CMO of iZettle tells me. “We had 40,000 users signing up during our first month in Brazil.”

iZettle has raised over $50 million since being founded in 2010, and investors also include Index Ventures, Greylock, MasterCard and American Express. Currently iZettle is active in the UK, Spain, Germany, Sweden, Denmark, Norway, Finland, Mexico and Brazil. The company would not respond to questions about whether it has plans to raise more money or what its international expansion plans are, except that it does have its eye on more growth.

Another part of iZettle’s evolution has been to work more with point-of-sale hardware providers to integrate its payment backend and also its hardware dongles into larger payment systems to sell into larger merchants or those who already had payment systems in place. The SDK will help iZettle integrate better with this latter group.

“We were happy to work closely with the iZettle team in developing one of the first tightly integrated payment device SDKs on the market, offering an awesome user experience that our customers are loving,” said Serge Sozonoff, Founder of iKentoo, a developer of iPad based point-of-sale systems.