I'd recommend window glass tint, it works way better than these shades. In Bulgaria it's legal to tint rear side windows, if you have side mirrors on both sides. Maybe the rear window too (but the law is not speciffic about it). You may check what the law in Poland has to say about tinting. There are two ways to shade a car window -- have it machined (or ordered new) with a special cover or applying a film tint on it. These films have also UV-protection, block a lot of IR (heat) too.
Here is just an example, the first website I found now:
http://www.llumar.com/en/Automotive.aspx You could search the net for a local company, I'm sure you'll find one.
IMO tinting is better than these fabric shades, you'll see a difference in A/C load and yes, it will be a bit more expensive, but it will pay off in time.