They could break out the transactions explicitly. That would help reduce the feeling that they're taking more than their share, if they're not doing that.
Well, if they're there to secure a good chunk of peanuts for themselves, for being the intermediator, many would find this shady and wont want to deal with them.
Sorry, someone donating $10,000 to their favorite FOSS doesn't want an intermediator to take in $500 just for making the transaction.
Even more so when the actual transaction infrastructure is not going to be their's anyway, but some banks or Stripe or whatever.