Payment Plugin - Order totals

Hey guys,

i'm developing a payment plugin and based on the user input on the payment info screen (number of installments), i need to update the order total value.

So far using the custom values dictionary i can calculate and send the correct value to the payment gateway but still need to update the order total with the new value.

I was thinking about extending the IPaymentMethod interface to add a new method (CustomCalculationFee) and overriding the OrderProcessingService, but maybe someone can point me an easier way to do it.

How about using the "Additional fee"?

THe "AdditionalFee" is calculated before the payment info screen, when the user select the payment method.

Here a picture of my problem:

https://imgur.com/Lm9E8C5