As you said, the service you're paying $100 for is to change each ticket. It is not a part of the price of the new ticket (which is what enables you to travel), it is the charge for exchanging the tickets.

In other words, you took the money you'd paid for transportation and applied it to a new ticket for transportation. That ends transaction #1, which is the transportation part - you surrendered one ticket for transportation and got another.

Then you were charged a change fee, not for transportation since that was transaction #1, but for the service provided when the old ticket was canceled and the new one issued.

Say you buy a ticket through Orbitz and pay them their $5 - $10 fee for issuing you a ticket. That fee has nothing to do with transportation - it is strictly a fee for the service Orbitz provided. If you wanted to change that ticket, would you expect US (or any airline) to consider the fee you paid Orbitz as part of the ticket's value to be applied to your new ticket?

Jim

ps - I see your new post. I tried to get the CofC earlier but it wouldn't completely download. Sounds like you had better luck.