When I went last year, between the time we booked our flight and the time we went, the price went down about $20 each ticket (there were 2 of us).
There were no charges to get credit back for us. They offered two way of getting our credit. A) apply it to a trueblue accout, or B) apply it to the credit card number that we used to buy the tickets (just to clarify, it wasnt a refund to the credit card, but they would link it to the account number so that next time a flight it booked with that number, they would apply the credit to your purchase).
From what I remember though, it was good for one year, and if it was on the trueblue account, you could trasfer it to another person some how....
But yes, call them or e-mail them about the credit.