Add To Cart - Sidebar

When you are on a product page, and click on CTA 'Buy it now', a dark overlay appears on the actual page, and a lateral side appears to let you see immediately which products are on your cart.

This way, you don't have to display it as a popup (Checkout now or Continue Shopping). The customer can either checkout, edit his cart, or click outside the side zone to go back on the product itself.

Photo credit: Mr Porter