When your purchase MagBucks for yourself, or for a friend, you will be sent a special code that will be used for a future purchase, similar to a coupon code. Just enter this special code at the Cart or Checkout page(s) and it will deduct whatever value is in your Cart from the total amount of MagBucks you have purchased.
If your Cart total is greater than the MagBucks value, it will use all of the remaining value of the MagBucks at that time. If your Cart total is less than the total MagBucks value, the remaining balance of your MagBucks value will continue to live on for a future purchase.
Currently, there is no option to use only a portion of your MagBucks during a transaction. It will always attempt to use the full value of the MagBucks for any transaction.