Any device that plays sound works: the shop computer, a tablet by the counter, a phone, or the desktop app for Windows and Mac from the download page.
Keep the device plugged in. Store mode keeps the screen awake while music plays, but switch off sleep in the device settings too if the music stops after a while.
On the per location licence, set the store up on the store's own internet (not a phone on mobile data): the first internet it plays from becomes the store's.