I don't see any developer documentation though - is this intentional, or did I miss the link somewhere?
No but seriously, I'm banging away at documentation right now - the problem is it wasn't clear what exactly we needed to document until we launched and got the deluge of customer feedback that we've now gotten. It's now become clear that we're missing some critical features (for a lot of people) and that's it's not clear how to integrate it. Live and learn!
Essentially though, for 1 to 1 payments, you'll just sign up, sync your bank account, and you'll get a JS snippet and an HTML div. Put those on your site, feed the JS a payment amount, and when someone clicks the button - they'll pay, and you'll get the money. Then, at the end of the payment, you'll get a payment ID that you can store and use our entirely undocumented API to retrieve information you need later.
I'm not making any excuses, but we spent the last year building, speeding up, and securing the engine that handles the connections to bank accounts and the system to process and guarantee "good funds" ACH payments. We had an opportunity to go to Launch 2014, and even though the front end presentation of everything to the user was, in a word, "fucked", we made the decision to go and do the best we could to make it usable. This thread alone has helped me find a lot of the "anxiety" points people are having, and the customers I talk to everyday are getting me more. You'll see Knox back on HN soon with a much, much improvement experience.