Very interesting topic in fact, I am not sure a unique and perfect solution exists.
In fact, it depends to how much you earn, how matter does for you the project, how big it is, etc. It is a question of feelings after all.
For example you may want to donate $20 one time to a useful tool you use, but for an app you enjoy using which match your own values you may want to send each year $50. But for some people it is complicated to give money, they need to satisfy their own needs before and people don’t have all the same incomes.
FMPOV, if the project is “just a tool” it can be a $20 one shot. If I use the software daily, it can be $50 per year. Maybe more if I feel it will help.
About the transaction medium, it depends. Projects can use Liberapay, others PayPal or Open Collective, or also in-app purchases. I don’t use cryptocurrencies because of the transactions fees.
Hope it helps!
Quite strange, I have my last publications posted here downvoted without any comment, and I don’t know why. This is kind of unfair because FMPOV it is just hyperlinks maybe interesting or useful, at least for me and other people I know. But it’s the life, it’s social networks, it’s people and I don’t care that much ✌️ Please, leave comments instead ✌️