This may be controversial, but have you considered offering a version of
Octave through the windows App store for a charge? This would be a
charge for convenience since users could still download from the website.
Richard
The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.
Can you just upload the "octave-5.1.0-w64-installer.exe" installer there or does it take some extra effort?
Otherwise I think that would be a convenient way of donation.
On the con side, I see that Octave would support Microsoft a lot [1]:
§ 6 b. Store Fees. 30% (non-subscription), 15% (subscription)
Kai