If all they need is an invoice and some papers, then it seems like a business-opportunity? Offer the service to manage their donations to OSS-projects, maybe offer some additional software for managing which OSS they are using and how much those need in donations. Seems like something the FSS should offer.

That may work for some businesses, but IME, the people who hold the purse strings generally aren't ok with just giving money to a project without getting something in return. Especially if it isn't a 501(c) non-profit they can get tax deductions for. I think it is actually a bit of a gap in tax law, because I don't think such a donation would count as a business expense either if you don't get something concrete in exchange.

You just described Github Sponsors, Tidelift, and a couple other less well known competitors. It's a business opportunity, but not a great one from what I can tell.