allow me to donate a set, reoccurring, monthly sum. I.e $20, 40, 60, etc, to 1 service
Just a note to add, a yearly donation schedule can also be good, as it can reduce payment transfer fees. But on the flip side, if a donatoe wants to stop donating, it's easier for a yearly payment to fly under the radar.
I think the best option is to offer both yearly and monthly options when possible