The DBC vouchers are like CASH; thus, if you buy a $100 ticket and use 2X$150, the price at the bottom of the receipt is still $300. If you don't take the flight and have to rebook, the full value stays with the ticket.

If you use a discount towards a ticket (e.g. PMO), that will reduce the total fare and the price on the receipt will show what the ticket was MINUS the voucher. So if you had a $300 ticket and used a $100 certificate to reduce the fare, it would show up as a $200 ticket. If you decide not to take that flight, the value of the ticket would only be $200 which would be applied to another ticket. These discounts can only typically be used once .