The Shelf is an app that we will publish on your own Apple App Store and Google Play channels, that you will need to open. You will directly get paid by apple and google, we don't take any royalties, only the setup and yearly hosting / manteinance. More info here https://shelf.pubcoder.com
Anyway, you don't need Shelf to sell books built using PubCoder. You can publish as a single app and publish on Apple App Store and Google Play, or export an EPUB and publish it on Apple Books or Google Play Books or Kobo store or any other book stores that support EPUB3 fixed layout format.
Or, you could distribute the EPUB file yourself on your website and get paid using Gumroad or similar services...