It used to be that if you used a gift card for a purchase and then cancelled within 24 hours you would get an e-credit for the purchase rather than the money reloaded to the gift card. In this way you could use the gift cards, get a refund, and then use the larger voucher(s) to purchase the ticket (and use more gift cards).

I've never tried this - I just know that a few years ago I got a voucher when I asked for a refund on a gift card purchase within 24 hours. Not sure if this is still the case today.