Thanks !

I see. Since the tarif is proportionate to the final price, the final price needs even higher than the initial price times (1 + tarif) in order to keep the profit the same.

Would you mind changing “instead of making, they make” by some other precise verbs? Your explaination seems very interesting but, probably du to my poor english, I feel like you saying the same thing over and over while changing the numbers and I can’t grasp your explanation.