I take Amex as a merchant and have done for 20 years. They USED to be about double VISA, but now are about the same - about 2% for me, and often small business gets here for 1.8%. Yes, that seems lower than many USA merchants are slugged.
Lots of car dealers here take it for a $100,000 new BMW or Merc. They eat the fee as Amex does special deals on big numbers - always has for me anyway.
The painting deal mentioned above is no surprise if you have even seen what Sotheby's/Spink/Christies charge you at an Art auction!!! Think
15-20% "Buyer Fee/Commission". And the seller gets slugged darn near as much most times, plus Insurance, storage etc. For a few % they can live with a credit card, do the math.
Not to mention the worldwide publicity they got for it .. we still are talking about it!
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Glen ~