You can add one or more pieces of digital content as "Extras" and then include them for sale in a "Package".
First, you should upload the content you want to sell in Packages. To do this, in the Book Tools menu for your book, click on "Selling & Royalties". Then, select "Extras" and upload your content.
Once your content is uploaded, you can add it to a package and sell it with your book. To do this, select "Packages" under "Selling & Royalties" in the Book Tools menu.
Then, you can add each Extra you want to the Package. It will then be for sale on your book's landing page. You can make multiple packages and sell them at different price points.
Here's an example of a book that also sells videos and other content in differently priced packages: https://leanpub.com/rprogramming.