[mainde]

>Moreover, I noticed that some merchants refuse my payment when I use e.g. Google Pay with my Amex instead of my MasterCard.

In my experience this is normally due to either how the card machine provider has set up the device or due to the lack of certification of the mobile wallet functionality on the "acquirer" backend ("host") that speaks to the card schemes.

It's annoyingly tricky to get the end-to-end transaction working properly across all schemes, all payment methods and devices. Different card schemes support different "payment kernel" parameters and have different certification requirements.

It could also be an attempt to save money on transaction fees, amex is generally significantly more expensive for merchants.

[Nursie]

Historically, Amex always required a separate retailer relationship and to act as its own acquirer. I don't know how true that is any more. They've just always been the awkward one, with higher fees and special relationships. Also they used to use ANSI standards on some stuff where everyone else used ISO... but that's going back 20 years!

[inkyoto]

Yes, that is still true – AmEx own their own payment processing network, and they do not allow outsiders into it, even banks they have the brand sharing agreements with, hence a separate retailer relationship.

[devmor]

>Different card schemes support different "payment kernel" parameters and have different certification requirements.

Those certification requirements are one of the biggest hurdles because they can change quite often, and unless you are a high-volume gateway, there may be no leniency for you, making simply refusing the transactions cheaper than processing them and being fined.

The digital cryptogram requirements for visa caused some major engineering expenditures for a few payment processors I'm aware of.

[EricE]

When I briefly had my own store I blocked amex because of their ridiculous fees. And they are pretty merchant hostile re: chargebacks too. The overhead and headache wasn't worth dealing with them. That was a while ago so maybe they have improved, but I still occasionally run into places that don't take them so I guess not.