Two fees of different natures
A marketplace fee has a percentage and a fixed part, and they behave differently. The percentage does not care about your currency; the fixed fee is stated in dollars and quietly distorts small orders — on a $6 item it can weigh more than the plastic.
Working backwards from what you want to keep
Most sellers think in take-home, not in listing price. The app turns the question around: name what you want to keep per item, and it finds the smallest price that leaves exactly that after fees, tax and cost.