2.9% is fairly standard with Stripe et. al., but fees from things like chargebacks could probably add up to another 1%.

Typical credit card fees are 2.9% + 30c. Assuming that they have regular fees at not non-profit rates(which tend to be lower), it would make their average donation amount to be around $7.14

Source: 48995/689239 = (x*.029+.30)/x

Maybe it includes unfavorable (normal for credit card xactions) exchange rate conversions?

As someone else pointed out 2.75%-2.9% is common, often thre is a charge per transaction too (on the order of 25cents after it's all said and done). The fee can change based on the card type (the merchant pays a higher fee on rewards cards normally...someone has to pay for rewards!) and international purchases can have additional fees. Charges backs can also bump up fees, ditto for outsourced fraud protection.