Seams like a rounding error. Is your price absolutely correct? You currently have it set to 1.00840. That isn't a valid price and is causing rounding errors. It looks like your intended price (which is what is displayed when rounded) is 1.00 based off the description of the plan stating "1,20 € (= 1,00 € + 19 % Mwst.)", which also isn't correct as that would come out to 1,19. You'd need 1.01 for 1,20. I've corrected the price and description to 1.01 and confirmed it works fine now. You should verify your other plans prices are correct as well. Anything over 2 decimal places can result in potential rounding issues.