Personally I think you’ll have a hard time enforcing this, it’s very much reliant on your clients being honest. Once people catch on that selecting B2C waivers the surcharge I think many will abuse it. Then are you likely to have that awkward conversation later to try to recoup the difference?
The only correct way to implement this is at the checkout level with your card service provider. At the point the client selects their card type, that is when any configured surcharge should be applied. Aside from SagePay, I don’t think many payment processors support this right now, although perhaps that will change in light of PSD2?