How do I add a "Favorite" to the list that already exists? Specifically, the Amazon.com webpage?

See the following from Customize your start page in Safari - Apple Support

Favorites shows the pages bookmarked as favorites. If you drag a favorite to another position, or drag it out of the window to delete it, your Favorites Bar shows the changes as well. 

Click that will take you to these instructions: Bookmark webpages that you want to revisit in Safari on Mac - Apple Support