Working in the PayPal Sandbox, when I purchase a product, after the purchase the cart does not zero out, the quantity remains showing on the cart icon. Going back to shopping and clicking on the icon shows the item just purchased still in the cart. Adding another product adds that item to the cart (now 2 items in cart) and when going to the cart the two items are there and totaled for both items. Sometimes, however, clicking on a different product prior to going back to the cart causes the cart to zero. This can't happen in the real world. A customer can't be charged again for an item they just purchased.