You can call in and ask the agent to price your flights flex, but do not expect the difference to reflect what you would see on AC.com between Tango and Flex. ACV is probably AC's second biggest customer, so they get serious discounts on seats, especially on charter type destinations where almost all the flight is on ACV.

As mentioned, the $300 upgrade to J on some destinations is a no brainer, but other where the difference is more realistic, Flex would be less than J, but not much.