Absa Vouchers

My son has a MegaU account from Absa, he received a voucher of R20 off for McDonald's. On Thursday I took him so he could pend his voucher only to be told by the cashier that they don't have an idea on how to enter the voucher so he could get his discount, they did however advise that they saw the email in regards with the vcouchers but they never paid attention to them and based on this my son was affected and I had to use my card to pay for him.