Go to the Actions Drop Down menu to the right of one of the Sold Items click on Send Payment Details. Both Purchases should appear on one Invoice. Change the Postage - do not Click on Reset.
You are Trading illegally on a Private Account which is just for Selling your own Second Hand Items from in and around your home. You need to Register with Ebay as a Business Seller. You are denying your Buyers their Consumer Rights. They could Report you to Trading Standards. They are entitled to a 30 day Change of Mind Returns Policy. Upgrade your Account by entering it into Help and Contact top left or at the very foot of the Page.
