I am curious how this ends up working out. Google takes a cut of the donations now... but the process of buying is probably easier. So maybe it will increase donation revenue, and make it easier to support free software.
(Taking a step back; it is pretty weird to have apps opening links to web pages that take your credit card number. The app store operators are right to view that with suspicion because I'm sure that for every legitimate open source project, there are a million phishing scams.)