How to setup additional cost per payment gateway?

I am writing via google translator, sorry

I have an additional expense configured: payment gateway for paying by card on the site – 3.5%.

The client placed an order with a promotional code, for example, a 10% discount.
Example of an order: 1 product in the amount of 5500 rubles – a promo code for 10% = 4950 rubles
Accordingly, in the additional expense for the payment gateway, I must deduct 3.5% or 173.25 rubles from 4950, and the plug-in subtracts 3.5 from 5500 rubles, that is, from the amount before deducting the promotional code, or as indicated in the order “Total by positions : “, not from” Order summary: ”
It would be very good to mend this moment

I hope google translator has translated it properly.