To be able to purchase using Apple Pay you must:
Have Apple Wallet enabled on the device you're purchasing with
Be using Safari
On the checkout page, tap the Apple Pay button or choose Apple Pay as your payment method.
To pay with a different card, tap the Next button or the Expand Menu button next to your default card.
If necessary, enter your billing, shipping and contact information. Apple Pay stores that information, so you won't need to enter it again.
Confirm the payment.
When your payment is successful, you'll see Done and a tick on the screen.
For more information, please refer to How to pay using Apple Pay online or within apps.