I see a "Pay with Apple" button (doesn't appear if you're not using Safari, I guess). I click it, and Safari pops up an OS-level dialog, prompting for TouchID / FaceID / Apple Watch / password authorisation. I authorise the payment, the goods are paid. Works on Mac, iOS, etc. The card is the same one I use to pay for stuff in a shop, so basically buying groceries and buying things online are like the same thing, one click/tap away. It's pretty neat, if you don't mind Apple being a middle man.