HeidiPay never charges interest or fees to your customers.
If you wish, you can charge an administration fee to your customers to cover the transaction fee paid to HeidiPay, but only for payment plans "Pay in 3x" and "Pay in 4x". It is forbidden under the Swiss Consumer Credit Law (LCC/KGG) for you to charge any fee to your customers for payment plans with a longer duration (Pay in 5x - Pay in 24x). Please refer to your merchant agreement for more details.