I buy stuff from Amazon on the desktop computer and read it on iPad and iPhone Kindle apps. So far works great for me. Dropbox is even easier, you really need to worry about paying for premium once a year, if that, more like once in five years when your credit card expires, so this is a non-issue.

In both cases I downloaded apps because I used desktop versions extensively not vice versa. I wonder if Apple's cut is a big deterrent for mobile-first strategy that was recently discussed in Fred Wilson's post that appeared on HN.