Perhaps I could have been clearer.
> Who adds these items to increase weight?
Private delivery companies add these items.
> Why?
Because due to the existing PMMS (Poczta Polska to be exact) monopoly on mail <50g, private companies would have to charge 2.5x the PMMS rate to deliver the same piece of mail. Adding the item allows a private company to circumvent the monopoly (by increasing weight >50g) and thus being able to deliver this piece of mail at a lower rate than PMMS. How this ends up being profitable is a bit of a mystery.
> If PMMS is adding this weight
They're not. They have the monopoly so they don't need to.
> they're wasting fuel delivering light items.
No arguing there...
This is what one of those metal plates looks like http://gfx.mmka.pl/newsph/222924/44881.3.jpg