You can use your Shine or Forever credit toward any purchase from your set.  To make a purchase, go to your Current Set page and click Buy Now on any piece(s) you wish to purchase.  Any available credit (Shine/Forever) will be automatically applied.  You'll see this reflected in your total after the purchase has been made. 

Shine Credit expires at the end of each month, so be sure to use your $21 Shine Credit toward something you love.  On the first of each month, you'll get a new $21 Shine Credit to use toward an in-box purchase.