Unfortunately, unlike instore purchases, you cannot get a “split tender” for online transactions (a split tender is where you to pay x amount from your AllGo Mastercard Gift Card, and the remainder using another payment source eg debit or credit card).
So, for online transactions, you need to make sure there are sufficient balance on your card for the full purchase price.
Please note, you must have 3D Secure enabled to use your card for online transactions – please see the detailed guide here.