You cannot use Travel**** or Ex***** to purchase unaccompanied minor tickets. An adult has to be traveling with them. I use Southwest who allows me to purchase online. The beauty is that it costs the same whether I buy 1 way or 2 one-way (round trip) or a round trip. Each leg has their own price. And the 'I didn't fly' policy is great. Don't know about the other airlines. Also, there is NO additional cost for the unaccompanied on Southwest.
A word of caution for those using Southwest (which is a great airline, don't get me wrong).
If the flight will be into or out of TX, you cannot book a direct flight. Gotta' love the Wright Ammendment.
AA has a $150 unaccompanied minor fee that can be paid either one way by each parent or all at once.
Last I checked, the big D and US had the same fees.
I have never specified whether or not the person flying is a minor. I just showed up at the airport with the flight info and filled out the unaccompanied minor form. I really have no idea how my ex used to book the tickets.