1. It doesn’t look like you are using an individual shipping price for the items. what shortcode are you using to display the “add to cart” button? Did you checkout this page:
2. There should be a link on the “order successfully completed” page that says something like “Return to Merchant”. When you click on that link it will redirect to the “Return URL” that you have specified.
3. Use variation control for this. More details here (see the “Using Variation Control Option” section):