Although I am no expert, I would think being charged twice for flights on the same day with your name on both tickets would count (unless the airline could prove you were in two places at the same time!). Documents to prove this to the bank could include the confirmation receipt from the airline or travel agent for both tickets and boarding passes that show you were on one flight (hence precluding your ability to be on another), as well as the details of your calls with the merchant telling you what to do to resolve or remedy the situation (names, dates of calls, summary of conversation, etc.).
Unfortunately, I've had to do this with a less-than-scrupulous merchant that sold me a defective product and wouldn't give me a full refund (despite violations of the Moss-Magneson Act and the Uniform Commercial Code). So the system works. Good luck!
So it's all -- well, BS, is about all I can say... I am sure each leg cost does depend a bit BUT it's a game they play, I think that is obvious. Even if they REFUND you eventually, the amount is going to be, well, what they determine. PLUS they get to keep YOUR money for SERVICE NOT RENDERED and get some INTEREST on it -- which they do NOT RETURN TO YOU.
In the end, I try to fly Southwest when I can. There are MANY THING I DO NOT like about SW. However their service has always been impeccable and I never had any issues -- in fact the opposite. Once I called to change a flight at the last minute, expecting a charge, and they did the oppossite, they got me a special and charged me for LESS...
Now go figure!
About the ONLY good thing you can say about the air travel market is there are lots of choices usually, so you can still vote with your dollar.