I'm implementing online payment on a website I'm working on. Everything is working on the technical side, but I have a UX question now.

Let's say the user added a product to his cart. It goes to the cart page that says what's in it and how much does it have to pay, and includes the checkout form as well. When the actual checkout is done (asynchronously via JS), the contents of the cart are checked again, and the total amount is recalculated. That means the user might have added a product to his cart in another tab, and this product will be included in the checkout even though it wasn't displayed in the cart page when it was generated.

How do you cope with that?