Do they account for the different interchange rates on different classes of cards within a processing network? Not every merchant will want to be that granular, but I think it is worth it for big ticket items.

I don't think most merchants have the option to be that granular. I believe that most payment processors only provide a simplified view of interchange rates (and other fees) to all but very large merchants.

For example Braintree is 2.59% + $0.49/transaction, plus 1% if non-USD and 1% if the card was issued outside the US.

They used to split transactions into two tiers, based on how much the actual interchange and other fees were, and you monthly bill as a merchant would show charges for each tier. Within each tier the charge to the merchant was a fixed percentage and a fixed transaction fee. There was no good way for the merchant to know ahead of time which tier a given charge would fall under, although they could guess that a high rewards card was more likely to fall into the higher fee tier.

They should A/B test this. My theory is that this creates some amount of friction. I was all set to donate $100, but now I have to go find my debit card or figure out some bank transfer nonsense and by the time I get back (if I get back) my motivation to donate has diminished.

Donation platforms ought to do some analytics on this. If I’m giving money, I want it to be simple without requiring me to second guess.

If a donation platform takes Apple Pay, the friction is even less. I don’t want to fill out your stupid long form, create a password, share my address or whatever.

But, I’m one person — this might be an interesting Masters thesis topic to study the behavioral economics of donation platforms.

What I've personally seen is a checkbox, you put in your donation amount and then you can check/uncheck "cover processing fees."

Found an example - https://donorbox.org/nonprofit-blog/donors-to-cover-processi...