Originally Posted by WineCountryUA

Seems still available, just tested
"Other forms of payment"
"Pay in person"
presents
"Select where you will complete your cash purchase (*Required)
Airport Ticket Office (service charge per transaction may apply)
United Ticket Office (service charge per transaction may apply)"

There may be some reason it isn;t available for your particular itin.
Are you close to the advance purchase deadline?
Have you redeemed an ETC or Travel Bank? (not sure if that has an impact)