Hi there,
The PayPal Express process directs the shopper to PayPal to complete their payment. From PayPal, they are then redirected back to the shopping cart to confirm their PayPal payment.

The thing is, what can happen on the shopping cart "confirm your PayPal payment" page, is the shopper can open a new window and browse through the store, and add more items to the cart.

If they do add items to their cart and then click confirm on the store's "confirm your PayPal payment" page that they kept open, they're actually billed for the additional items that they added on their other web browser window.

I think that is a bit of an issue, albeit minor. Perhaps the shopping cart could maintain some sort of state so that it knew that the shopper was in process with completing a PayPal express payment, and prevent the shopper from adding more products to their cart until the PayPal payment was complete, or until they opted to cancel the PayPal payment.

Not sure if that is an issue for any stores, but I could see it being an issue.

I like what happens on the confirm PayPal payment screen if the cart is emptied. It displays an error message saying that the order was already placed or the cart no longer exists. Perhaps the same could be done here if any items were modified in the cart as well...that might be the easiest solution....if it's considered an issue!