That understanding is incorrect. CC companies always get their due, its just a matter of if the vendor accepting the payments chooses to offset with a fee. On some transaction types, including donations, assessing a fee is illegal so the merchant has to eat the costs.
I've worked in accounting on both the issuing (bank that offers card used to buy the product) and acquiring side (merchant accepting payment) of credit card transactions
The acquirer (viva, 7/11, any store) will pay different amounts based on their size and negotiating power. Big players may only pay like 1.5%, cant really say how expensive it is for smaller ones (but I think upwards of 5% or 6% as you said). This is what makes square an attractive product, they stand in as the acquirer, and small vendors pay them 2.75%, and square aggregates all their volume to only pay like 1.5% on the back end(assumedly, allegedly)
About 1.1% of that goes through to the issuing bank. Visa/MC and other networks take the difference.
(sorry for a million edits, i know thats not a good look, but just trying to get the message right)
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