I feel like it shouldn't be poor form to say on this site - a site that predominantly has been about building tech companies and revenue streams - to get over it and charge them.
A German business on the other side of the transaction rarely will pay anything if there's not a proper invoice (listing net sums and VAT separately) on file. And they usually also require the (business) tax ID of the other party.
To be fair, founding a business is a matter of filling in some form, paying a small fee, and a few days of time (depending on the workload of your local trade office). But still - if this is a one-off thing and you don't even know whether there will be more... I'm not sure I'd want to go through the hassle. Especially if it means having to hire an accountant with monthly costs, when I don't even know whether there will be more income.
More than enough has been written about how the ecosystem in various EU countries stifles entrepreneurship. It's not new information, but it's not a problem for the vast majority of people reading this site. This site is also focused on supporting entrepreneurship, we should be supporting the default approach of getting paid for your work.
Germany needs to change, not the stance of this site or the argument of charging for your work/time/etc.
[1] (the proverbial you)
I registered a private SW development entity in Germany easily, without any costs. You just need to declare it as System SW, not Applications, or you'll have to pay Gewerbesteuer and Kammermitgliedschaft. Which is a lot and not needed at all. Your yearly taxes are trivial, no need for an accountant.
Alternatively, people could just stop complaining about it.