The introduction of fines for owners of cars with European license plates has been postponed, but the problem has not gone away. Ukrainians come up with all sorts of things to avoid punishment. Some get rid of cars in an unusual way, while others disguise them. 
This Mercedes-Benz E-Class was decorated with OSCE logo stickers. Probably the owner thought that in this case the police would not bother him.
Accordingly, there is no need to clear customs for a car with European license plates. 
However, to be honest, we doubt that this will help. Everyone understands perfectly well that OSCE representatives do not drive rusty Mercedes, but more expensive cars, often with diplomatic plates (and not Polish ones). 