I usually just leave it as a credit for future travel given the small amounts of money. The website is actually very good about letting you use the credit during the payment process. You can stack as many as you want, and you can use it when you book for others to pay for their seats and bags and such. The only problem is that they do not track your credits well. I have to keep a list.

Three of my mileage runs last year were either cancelled or changed to thru Denver with an over-night. So, get used to Frontier!