I have done this each of my last four trips including the one on 2 weeks. We have a local chain GIANT EAGLE that has gift cards, and each $50 in GCs you buy you get 10 cents off a gallon of gas. Sometimes they double to 20 cents. Driving an F-150 with a 30 gallon tanks this really saves, I have gotten average 2-3 free tanks of gas this way for each trip $200-300 each time.
If you cancel they need to put the money back on the cards so don't discard them until you go, or they will put the money into escrow until your next trip if you reschedule.
When I book I always do it over the phone, something new is they can only take 2 GCs a day by phone, so just call in the next day and pay more.
I have never had a problem with this, even when I had 20 $100 cards I had to use(Giant Eagle was out of the $500s).