IME, it’s less about POS systems and more about what the merchants are willing to give up in fees.

Amex charges significantly more than the others, to the point that multiple businesses I frequent offer a sliding scale - par for cash/check, +2-5% for Visa/Mastercard/Discover, +5%+ for Amex.

This is just one more input to an optimization problem for me, but for merchants makes a significant difference: my mechanic says he saves thousands of dollars a month by turning Amex customers into Mastercard customers, and Mastercard customers into cash customers.

Well, I’ve noticed businesses switch to accepting Amex when they got one of the newer POS systems. I wonder if Square/Clover/etc. have negotiated different rates for their customers.

I think Square has a flat fee no matter what card. They probably just average out the fees between the cards.