"Fund" link should have a default payment for apps already installed

Not sure if this is a feature or a bug. For apps already installed, the "fund" link is the only way to pay without uninstalling and reinstalling. I believe a "suggested donation" that is equal to the app developer's default price would be appropriate here for at least two different scenarios:
1. those of us who "try before we buy" to see whether an app will actually work.
2. I also have a backlog of apps that I downloaded for free simply because the payment system wasn't working at the time, and I'd like to give the suggested payment for them without having to uninstall and reinstall.

No files or tests are identified. Start by tracing the existing "Fund" link and the payment flow for installed apps, then compare it with the app developer's default price; done means installed apps offer that suggested payment without requiring an uninstall and reinstall.

Written by the indexing model from the issue text.