The already paid package is used when you made a photoshooting with your customer and included some free (digital) pictures in the price for the photoshooting.
It is important to remember that the already paid package can only used ONCE per gallery! So it would make sense to use this function for a photoshooting with a single customer and not for a wedding shooting for example.
To activate this function you need to go to the overview of your gallery and click on "Shop":
Here you need to activate the digital shop. Then you can already select an already paid package from your digital pricing list: