Thanks tentseller.
For anyone thinking of buying the discounted ticket through Hong Thai, at the cost of my mistake
As Iamhappy suggested, buy the tickets in separate orders and only when you know you need them due to the short expiry date.
For reference, I purchased the tickets on September 8, 2015. The expiry/validity period is marked November 7, 2015, so just about 60 days / two months (give or take a few days here and there.)