But it’s always a way to help open source projects just make a personal donation, as a developer, because our money is, in fact, the money our customers pay us (freelancers, for example).

Yes, I know this sounds a bit odd to contribute your own money on behalf of the clients, but when you factor it in as a (explicit or only mentally) as a line item in your bid, then in the end it is the client after all who pays for it.