I would maybe google search other artists or similar establishments that offer gift card to help you get the wording right…
https://www.artgallery.co.uk/gift_vouchers/
https://www.paintingwithatwist.com/giftcards/
How about this from Painting with a Twist:
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Gift cards do not have an expiration date. Gift Cards are non-refundable and have no cash value.
**