You have the luxury to be able to spend your card with any Mastercard accepting retailer. But you must spend to the balance of the card.
If you are trying to spend online and the value of the item is more than your card balance, then why not use you bank debit/credit card for this higher value item and your TPG card on something more mundane, that you would have otherwise used your bank card for, ie. an online grocery shop?
If you’re able to use your card in store, then simply pay with the difference with your bank card / cash and ask the cashier to ring through the balance of your TPG Gift card.
Also, don’t forget to sign up to EAYS, it’s FREE to join and you get 3% return: