Note: This feature is currently only available on iOS.
Make sure that you have created the item in your Product Library.
Slide your finger from right to left once on the main page of the app (pictured below).
Tap on the item you wish to add (pictured below).
The quantity along with the price will then be included at the top of the screen (pictured below).
For more information on how to select an item via the Tappr App, please watch the below video.