Change or cancel flight? This is a simple problem, but I wanted to hear an answer from someone else.
I had a round-trip flight that cost 135 dollars for a recent trip. At the last minute, plans changed and I decided not to go. I called the airlines asking to have 135 dollars of credit added for future use. The rep told me that there was a 200 dollar fee to do so, so I would be better off just pulling a "no show" and losing the 135. Which is the better option, no-show or credit, assuming I'm 100% sure that I will book a future flight?
 A: If you want to decide between two choices, it is often simplest to compare the possible situations after you have seen the effects of your choices, and not start calculating "losses" and "wins" because that would just complicate things too much and make it difficult to communicate you thoughts.
Let us say you have $B$ dollars in your bank account after paying 135 \$ for the first flight. Let's see what you have after two flights if the ticket for the second flight costs $x\geq135$ dollars.
Option 1: No-show. After the first flight, you have $B$ dollars, after the next flight, you have $B-x$ dollars.
Option 2: Credit. You pay the fee. After that, you have $B-200$ dollars and 135 \$ worth of credit. After the next flight, you have $B-200-(x-135)=B-x-65$ dollars. So you have lost 65 \$ compared to Option 1.
What if $x<135$?
Option 1: No-show. After the first flight, you have $B$ dollars, after the next flight, you have $B-x$ dollars.
Option 2: Credit. You pay the fee. After that, you have $B-200$ dollars and 135 \$ worth of credit. After the next flight, you have $B-200$ dollars and $135-x$ worth of credit. If you are not going to fly for a third time, you have lost $200-x > 65$ dollars compared to Option 1. But if you are going to fly many times, you'll eventually be able to use all your credit, so again you have lost 65 \$ compared to Option 1.
A: It should be quite obvious that the rep is right. Unless in an era of extreme deflation, paying 200\$ now to have 135\$ available at some later point in time is a very bad business model. (Also, I assume the airline would not pay you any interest on the 135\$ deposit)
A: If you pull a no-show, then you have a net loss of $135$ wasted dollars.
If you add the credit for a future flight, then you do not waste the $135$.  However, you waste $200$ instead.
So the no-show is better.
A: If you do a no show you lose what you paid, 135 dollars
If you want to have 135 dollars in the form of a gift card, you would have to pay 200 dollars to use the 135 dollars. So now you flying for 200 dollars instead. It's like exchanging a 135 dollar ticket for a 200 dollar ticket.
A: Here's a way of thinking about the problem that might help.
You put some money in an envelope and give it to me for safekeeping.  You change your mind and ask for it back.  I refuse to do so unless you give me $200.  Should you open your wallet and hand over the ransom in order to get your envelope back?
