Over the past year, when I’ve asked many of the startups I’ve covered– including SideCar, Postmates, Shoptiques, Exec, and Lyft–how they integrated a payment system into their sites and apps, they’ve all mentioned another SF-based startup, Stripe, as their solution.

Stripe is the brainchild of two brothers from Ireland, Patrick and John Collison, who up until a few years ago were, in younger brother John’s words, “two kids in Ireland building software people could use all over the world. That was incredibly easy to do. But charging money for it was really hard.”

So they set out to “turn financial products into code,” for both the web and mobile devices, which in due time became Stripe.

After an extended period (18 months) in private beta, the company launched publicly late in 2011. Since then, growth has been strongly viral, spreading largely through its APIs and the developer community.

Today Stripe handles millions of dollars of payments per day from thousands of businesses not just startups but the likes of Walmart, NBC and Millennial Media as well.

As for the highly encrypted security system used by the company, it is certified as a Level 1 PCI provider, which is the highest level available.

Stripe’s business model is based on a straight transaction fee of 2.9 percent plus $0.30 per successful transaction. It does not charge its partners a technology-licensing fee.

“The trend is for us to spend more and more of our lives online,” notes John Collison. “It’s such an exciting trend it would be terrible to have more barriers to progress. We want to make it very easy and cheap to work with us. As our partners grow, we will earn more revenue later on in the longer term.”

The company of 47 is moving its headquarters from SoMA to the Mission at the end of this month, where its neighbors will include SoundCloud, and the non-profit Electronic Frontier Foundation, which uses Stripe as well.

Stripe is currently in private beta in the UK, which will be its second international market – until now only businesses in the US and Canada have been able to use the service, although payments can be processed globally.

Prominent investors, including PayPal co-founders Elon Musk, Peter Thiel, and Max Levchin, Sequoia Capital’s Mike Moritz, and other top investors in the payments space back the company.