I'm experiencing a similar problem. I purchased a ticket on A3 from ATH to JTR on united.com, as I wanted an 016 ticket to earn PQD on UA. When I pull up my flight on A3's website, there's no option to purchase baggage. I called A3, and after several minutes of transferring me between various departments, they told me that I can't purchase baggage in advance because my flight is on UA ticket stock. They said my only option was to pay for the bags at the airport at the walk-up prices. If I knew this was going to happen, I would have ordered the ticket directly from A3, as the PQD earned on the flight isn't significant enough to offset the higher bag fees.