"... it was to offset the costs of the payment processor, which if you've never had to deal with the likes of Visa, Amex, and Mastercard, generally is between 2-5% of each transaction. (i.e., every card swipe) "
Yes, they do.
However, Patreon charges the payer 1 transaction for all their patreon pledges.
But they are going to add a charge to cover this per pledge not per transaction.
So if you pledge Doc $100, you will now pay $103.25.
If you pledge 100 patreons $1, you will now pay $138.
Plus any local taxes of course (I get to pay an extra 20% on top of the pledges, so a $100 pledge total would cost me $120 last month, and this month will cost $165.60... plus exchange rates, of course)