You didn't miss anything as the seller pays the fees and as a result raises the price to cover the fees.
The transaction fee that I referred to above was using the Amex card in Canada and it's added to my CC bill. This is in addition to whatever fees the seller pays. And no Canadian transaction fee on my bill when I use my Visa cards.
However from what I've read paying in cash these days is actually more expensive for the seller than CCs. The more cash a seller has the higher the security risk and money is spent on safes, armored cars, etc. So the merchant may make LESS with a cash purchase.
I get at least 1% cash back on any purchase using my CCs. And 5% on fuel, groceries, etc. 2% on travel including CGs that take Amex.