OK so, I'll get this question out of the way early: why not use Peek & Pop?
Seems like a natural fit with the way you're using the "card" view to inspect purchase details. I assume this design was started before the 6s launch, and legacy iPhones deserve the support... but I figured it'd be inevitable that you'd be asked this anyway. :P
And, of course, the obligatory "I love the Stripe design team" comment. You guys are seriously great. It would be so easy for a payments company to have overly stark, lifeless design and Stripe continues to inspire with their continued focus on truly great UX and visual design.