With the paper vouchers take a picture and save it somewhere the ink will ware off on the vouchers overtime so be careful
as stated the best option if you want SW to keep track of the voucher in your account is book a flight then immediately cancel it. **Keep in mind if you book a WGA flight you will lose the transferability so if your not sure if you will use it or someone else book a WGA+ fare that way it would remain transferable.
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