Do I need to use my Reward Wallet balance all in one go and does my balance have a expiry date?

You can use however much you want, when you want and funds in the reward wallet do not have an expiry date. When the e gift card is completed each retailer will have their own terms and conditions which will include expiry of e gift cards.
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Related questions
• What is Reward Wallet and what are the benefits of using it?
• Is there a minimum or maximum amount of cashback I can transfer to Reward Wallet? Are there minimum and maximum thresholds for the e-gift cards within Reward Wallet?
• What e-gift cards are available in Reward Wallet and What happens after I’ve requested an e-gift card from Reward Wallet?
• How can I access my Reward Wallet and does it cost to transfer to Reward Wallet?
• Can I access my Reward Wallet on the app?