The voucher is applied to the total of the flight cost, as the airlines allow you to use the value of the original ticket to rebook to a new flight; but, if the new flight is cheaper than the original one, it’s unfortunately not possible to keep the residual amount of the voucher and use it later. The airlines accept only one rebooking per ticket and passenger. 

Likewise, we are unable to cover any extra services or products with the residual amount of the voucher, since the airlines will not refund the residual amount.