While I would agree that quoting Interchange+ is 'in a way' more transparent - in that it shows you the banks markup, it also doesn't cap your costs.
There are over 400 Interchange rates, varying by card type and
transaction circumstances. It will be possible, for instance if someone uses a platinum, airline rewards card for you to pay 4.86% + .22 in interchange before they take on the .16 or .20 'profit'. Banks are switching to this approach because it guarantees they make money all the time - on every
transaction, which is simply not the case in most instances.
My approach is to just tell you what your cost will be and then I take the risks on what the balance of interchange rates will be. Sometimes I win, sometimes, well not so good for me.
I'm sorry if this sounds confusing. I'm really thinking of putting on a traveling roadshow, explaining how these fees work. While most of the processors working mainly in the carwash industry are good, there are some mainstream banks that are really taking chunks out of people through this cost+ approach or by hammering you on all sorts of additional fees. All of the fees should be stated in the contract.
If it's not spelled out, you aren't paying it - that's my motto.