You add upload the card and add another check-out step "confirm shopping cart" which shows what's in the server session. And btw: wouldn't you rather store the shopping cart in the server session from the beginning? The cart would survive browser restarts this way.

You can upload the cart to the server and add another check-out step "confirm shopping cart" which shows what's in the server session. And btw: wouldn't you rather store the shopping cart in the server session from the beginning? The cart would survive browser restarts this way.