We have digital add ons and physical add ons. Anyone can add digital add ons to their pledge by simply adding the appropriate amount of money to their chosen pledge. For example if you have already pledged at the $45 level for a hardcover book and a PDF in 5e but you want another PDF in Pathfinder as well you would add $5 to your pledge and that credit will show up in Backerkit where you will then indicate that you also want the Pathfinder PDF.
If you've backed the project at a physical level of $25 or more you can add physical add ons to your pledge by adding an extra $10, $25, or $40 to your pledge depending on which physical add on you'd like.
If you've backed the project at a $15 digital level you cannot add physical products to your pledge. This is due to the fact that at the $15 pledge level you are not paying for shipping. In order for this project to be a success we need to collect shipping fees for physical products and that means digital backers are not allowed to add physical products to their pledge. Instead if you want a physical reward, change your pledge level to your desired physical reward and add the digital reward to that.