Hey again,
thank you both for your detailed answers!
Well… I have been confusing the replacement back cover with the back case so I thought there’s not available directly from Fairphone. 
But, well… almost 40 €!? For a bit of a piece of plastic? Almost 10% of the price of the complete phone, really!?
That’s not really sustainable, putting such a heavy price-tag on something that should be absolutely included with a Smartphone especially one that says that sustainability (no matter if for humans or nature) is the most important thing. I mean: How sustainable is it if I drop my phone one time and then I’ll have to get all kinds of replacement parts for it because its back wasn’t properly protected by default??
I’ve read here that the fp3 comes with a bumber: (Looking for) FP3 Bumper - Well… why not the more expensive version of it!? 
So I’ll basically have to pay 510 € for the phone (instead of the advertised 470 €…). That makes me preeetty angry and most of all disappointed at the moment.