"Attach" means charging for something outside of the app store. Dropbox, Uber, Square all do it. I remember seeing a Chris Dixon post about this on HN a few months ago.
My take-away was that I wanted precise pointing and a keyboard and a big screen for those tasks, because I'm fussy about the results.
But like a lot of the HN audience, I'm not very typical, so my experience may mean little ;-).
Regarding payment model, payment wasn't the issue. It was functionality.
Turns out that to get the lines straight, group/ungroup objects, line everything up, choose fonts and colors, etc., etc., it takes a lot of input that is not particularly well-done using touch.