Maybe I need caffeine, but I don't follow your comment.
Who adds these items to increase weight? Why? If my item is <50g, I use the Polish monopoly mail service (PMMS?) and pay 1.55pln (40% of what I'd pay with the competition, according to your numbers.) If PMMS is adding this weight, the mind boggles - they're wasting fuel delivering light items. If customers are adding the weight, why would they want to increase their costs to mail light items?
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.
Who adds these items to increase weight? Why? If my item is <50g, I use the Polish monopoly mail service (PMMS?) and pay 1.55pln (40% of what I'd pay with the competition, according to your numbers.) If PMMS is adding this weight, the mind boggles - they're wasting fuel delivering light items. If customers are adding the weight, why would they want to increase their costs to mail light items?