Hi StandingSam,
Did you buy a ticket for a journey or a subscription? (‘Dal Voordeel’ is a subscription)
For journey-tickets it usually isn't possible to get a refund, because you might travel with it anyway.
For subscriptions there is a ‘14-day cooling-off period’. You can cancel the subscription in this period by contacting the Customer Service directly by phone (+31 (0) 30 - 7515155), chat, Facebook or Twitter. You can also submit a form. More information is found on this page.
Single/return online tickets for the day with ´voordeel´ are ONLY allowed to use if one travels with someone who has a permanent subscription on his/her card.
DalVoordeel cost 5,10/month and with that you get 40% off when checking IN during the off-peak hours. Must be paid advance via I-Deal (hence via a dutch bank-account) and is always loaded on a chipcard with your foto+name on it.