We've used
UT- great service. You pay the price posted, unlike some secondary ticket sellers who hide tax, and shipping fee until you go to check out. And sends you vouchers, not real tickets.
UT doesn't charge for shipping, tax is incl in the price posted, and they send real tickets.
We upgraded our tickets to an AP after using the ticket once or twice. As long as there are days left on the ticket, you can upgrade it while you're there.