I am sure others can provide far more details for all of this, but I just wanted to pass on my recent experience with redeeming Southwest credits...basically was a royal pain.
I have a party of 7. We earned two credits for each ticket for various reasons...one $25 credit and one $120 credit per person. I was attempting to book another flight using those credits.
I was trying to book a flight that cost $139, and even though I had over $1000 in credits in my account, I could not do it without having to pay additional money. What?????
Apparently newer operating procedures for redeeming credits only allow use of 4 credits each time and the credits are selected for you. So each attempt I made the max credit I could use was $100 because that was my first 4 credits of $25 each. It was such a mess!
The good news is that the people at Southwest on the phone were AWESOME! I cannot emphasize enough how good they were with a complicated situation. It took numerous phone calls and several hours but in the end they booked my new flight for all seven of us with my credits.
Hope the booking of using credits goes far more smoothly for you, but do not hesitate to call Southwest. Oh, and they also have a way cool call back feature instead of sitting on hold - sure wish Disney had that!