Once you select and place the items you want to purchase in your Cart, you will see the Pay with Amazon icon inside the cart. You can view the cart by clicking highlighted "View your Cart" in the items page (photo #1) or by using the down arrow next to the paddlock with the items in your cart (photo #2) and clicking view cart and checkout.
In the cart page below the checkout and quick checkout links you will see the "Pay with Amazon" icon. (photo #3)
When you click on the "Pay with Amazon" icon you will be taken to the secure Amazon page where you can log in to your Amazon account and pay.