The Apple Pay Documentation states:
Websites that support Apple Pay can display this icon in a summary view and in the payment sheet of the connected device that’s used to authorize payment. The icon provides visual reassurance that payment is going to the right place.
The icon they are referring to is the blocked "A" you can see in this screenshot.
How do you set this up? I can't find any technical documentation for that anywhere, but I assume it's supposed to be passed into the ApplePayPaymentRequest somehow.