It should be 10% extra if you pay with fiat. Perception matters, ypu charge more for the shitty money that loses value, not less for the good money you want.
Discussion
✔ 10% inflation and exchange surcharge
For me there is kyc payment methods and non-kyc ones. Cash is fiat but it's still far better than card.
More importantly, merchants should always add the ~3% (or, depending on their location and risk profile, up to 9%) card fees on top of the final price if someone wants to pay with a card. People think it's all free, and most aren't even aware that the merchant pays for it.