For example, in Spain you aren't allowed to extend invoices if you aren't first registered as an autonomous worker (freelancer), which most people aren't because they already work for some company. But even if you did, then you'd have to pay for the obligatory social security for freelancers, which is around 260 Euro per month (so staying registered indefinitely as freelancer is not financially sensible, if you are not really doing actual contract jobs).
I would think this is similar in most other European countries, which in other words means that for the most common of the FOSS developers, writing a legally valid invoice is a no-no.
Of course the matter changes if the company doing the donations doesn't really care about the invoice being legally correct inside the country of the developer. But I wouldn't advise the developer to do things like that... (s)he would be risking a fine.
Of course we could say that "this will never actually happen", but if the company had to pass a tax inspection, they would have to prove all their expenses by showing the corresponding invoices. And this would show that the developer has been generating irregular invoices.
I don't know for bigger companies, but as a (proper) autonomous worker in Spain, I had worked for just 1.5 years and already was asked once to prove some expenses with the corresponding invoices.
Not a single company could pay such an invoice.
(Germany, but pretty sure this applies in most of Europe)
If you can read spanish, articles such as this one explain the options: http://gestron.es/ventajas-ser-autonomo-trabajar-cuenta-ajen...
I suspect that if you started getting any reasonable quantity of donations that would also pique the taxman's interest.
(Fortunately the paperwork is very minimal: Basically, you add an extra one page form to your annual income tax return.)
Even professionally, shops with good admins/devops will usually prefer CentOS/openSUSE over RHEL/SLES, since they can handle almost all the support themselves.
The other model is tools like Gitlab/Alfresco/Magento where the enterprise version has closed source code not available in the OSS version.
I wrote a thing about different OSS support philosophies a while back:
http://penguindreams.org/blog/the-philosophy-of-open-source-...
And shops with large enough IT budgets and huge downside risk (like finance) will pay for the SLA guarantee because ultimately it's really cheap insurance against the cost of catastrophic events.
I've been in banks where the admins were the definitive authors of books written in their space, but where CentOS or RHEL on a server was chosen purely on the basis of the value of the service being hosted.
There's also an example of software which clearly doesn't wish to follow such scheme: IDA Pro.
(Obviously you want that agreement in writing, but start by just getting conversations with users who would be qualified for this. Anybof your users with 50+ employees could be good candidates.)
So basically, yeah, just Joe the accountant from your local strip mall.