Firstly thanks for your desire to donate, unfortunately I cannot put anything else in the app other than a play store payment because if I do the app gets pulled down very quickly (I once put a Bitcoin payment address in and the app didn't last long).
You can contact me at
support@macrodroid.com for other options.
The problem is that there is no obvious mechanism on the Playstore to enter an additional, optional amount as a tribute to the development of MacroDroid. Do you have a mailing address to which I may send a bank check?
I'm very surprised to learn that there are few developers working on the MacroDroid project. I had envisioned a team of staff, designers, developers, and quality assurance. Thanks for reading and responding to my comment. I'll send you an email. -joe-