Just adding to the chorus here. I tried to book a trip on the website from the U.S. to YYZ (Toronto) and the Travelbank payment option failed to appear. All flights were UA or UA Express, with one connection each way. The flights were to be ticketed in USD for U.S. point of sale.

Just for fun, I tried booking a U.S. domestic trip and an international trip (U.S.-ZRH). Travelbank appeared for both of those itineraries.

I gave up and called. The agent was able to book the YYZ trip using Travelbank, and waived the telephone booking fee. She didn't know why the same booking couldn't be done on the website.