Tell the buyer to add the items they want to their shopping cart then click the link on the top right to request total. That will send you a request for an invoice where you can adjust for the combined shipping rate before the buyer pays. Tell the buyer this option is only available on the website, not the app, and if they don't have a computer they can use their phone's browser in classic view.
Another option is to set up automatic combined shipping rules. If you're interested in going that route come back to let us know.
