I expect that for a lot of projects/people it wouldn't be hard to send money. Either the project already accepts donations and sends those to maintainers, the person already has donation systems setup, or the person doesn't need the money and ask you to send it to a FOSS charity (like Software Freedom Conservancy) instead, or you send the person and email and they go sure, here is my PayPal/etc account number for transfers. Personally, I'd be very happy with money for paid support though, in my experience its pretty hard to extract money from most open source projects, basically no-one wants to give you cash because the project already meets their needs.

https://sfconservancy.org/