I would rather the default be accurate for the majority of my donors and my org cover the difference of an Amex fee than my donors overpay fees. Please either change the default to the Visa/MC fee of 2.79% or make your donor fee smarter to cover the appropriate fee by card type.

Why doesn't Blackbaud Checkout use the BBMS rate of the card type?

At the time when donor cover is presented, the payment workflow in Blackbaud Checkout does not know which payment method donors will choose. In order to simplify the Checkout workflow and avoid any confusion during the payment, we use a fixed donor cover rate for all transactions.