Yes, promo codes and GCX Cash can be used in the same order to purchase discounted cards on the GCX Platform. However, promotional codes cannot be combined with GCX Cash when purchasing CashBack cards.
Only GCX Cash can be used towards a CashBack order. Using GCX Cash on a CashBack order alters the earnings amount because the CashBack rate is based on the amount paid out of pocket, not the card value. The applied GCX Cash deducts the out-of-pocket price which may deduct the amount you will receive back.