Fairware is open source software with the assumption that enough users are fair for developers to expect compensation for the time they invest in the project. See the About page for details.
When a contribution is made, it is allocated to unpaid development hours in order in which these hours have been invested (The oldest unpaid time log is paid first). A single contribution can be allocated to multiple time logs and a time log can be compensated by multiple contributions.
Some projects have dependencies. For example, the dupeGuru project depend on the project "Hardcoded Software", which represents time being invested in Hardcoded Software but that isn't related to any specific project (in short, overhead). Timelogs from these dependencies are simply merged with timelogs from the main project when comes the time to allocate contributions.
I gave myself a 35$/hour rate on the basis that if I worked for someone else, I'd get something like 25$-30$. However, I also have business expenses, which I try to factor in that hourly rate. I estimate it to be somewhere between 5$ and 10$. Hence, the 35$ hourly rate.
Nope, and that's what so cool about Fairware! As Fairware developers, once we've been fairly compensated for our time, we're happy. Since we don't believe in intellectual property, we also don't believe that you owe us anything by using the code we created for which we've been fully compensated. We're just glad you like it!
You don't even have to, like with typical "donate button" open source software, feel guilty about not contributing. If all hours are paid, everything is cool.
Of course, nothing stops you from contributing even though all hours are paid. If you do, your contribution will stay unallocated until a developer invests some more time into the project. If, for example, you've asked for a new feature in the application, adding a contribution to go along your feature request might make your feature come faster.
Most download websites only differentiate between "Shareware" and "Free" and they don't have an "Open Source" classification, which is a problem with Fairware. Fairware software is submitted as "Free" to those sites because Fairware is closer to that than to Shareware.
With Fairware, it's not the right to use the software that is sold, it's the time invested in it. Once all hours invested in the software have been paid off, nobody is asked to contribute, so a "Shareware" classification would be wrong.