Actually the checkout experience is a small factor in Amazon's success (in online retailing). The main factors are 1) Big selection 2) Fast and convenient delivery 3) Low prices. The checkout experience just a small subset of point #2.
We're not here to solve everything around online commerce, at least not yet, But, the payments stack (checkout, fraud, internationalization, etc) is really difficult and results in substantial lost revenue. By helping convert 10%-50% more orders, we help businesses grow faster. That's our very simple goal.