> Or just make the tips refundable for a short time period with no formal process
Then you get the fraud the other way. People get content and then ask for a refund.
> If people abuse the system, ban them
So now you're back to needing an expensive dispute resolution process.
> If sites frequently get refunds requested, disable the app for them.
Which punishes the sites who are the most common victims of buyer fraud.
There are three options. Option one, dispute resolution process, high transaction costs. Option two, caveat emptor, no refunds. Option three, caveat venditor, totally unconditional refunds.
And option three doesn't really work either, because if readers are anonymous they could all ask for a refund with no consequences, but if they're not it's a privacy problem. So what you really need for small transactions is option two. Which isn't really a problem for small transactions, because they're small -- if somebody rips you off to the tune of $0.03, whatever, lesson learned, don't do business with them again.