For a general overview of selecting things and operating on them, see video I linked here:
How to Video: Repeating Group (RG), Selecting an Item, and Showing it or Operating on it in a Popup / Pop-up (on vimeo: https://vimeo.com/289114359)
Watch the above video first.
Continuing on with my silly store demo, I show how to do this using List Shifter (but the general technique doesn’t need a plug in or anything). Def watch the video above first to understand the basic concept and then see how I make it a little easier in the video below: