I am so happy that you had a great resolution to your problem. I have found that US Airways CSR's often go out of their way to see that a problem is resolved satisfactorily - so it is really great to hear confirmation of that.
How can I get the name of the agent to send her a Thank You note?
The first thing I do when talking with CS is to note the name of the agent. It helps to know it when things a)do go your way and you want to write a thank-you letter and b) it's always helpful to call the person by name when on the phone with them. I have had some great conversations with CS reps!
A letter to Customer Service would definitely be appropriate in this instance - and, as stated, hopefully the right person will get the kudos!
Thanks for sharing.