The flex class offered on AC is the lowest available (usually K, L, or T for domestic/transborder flights). You can call AC to request a different class, but it will cost more. The difference between each class is usually about $30. Therefore, the highest classes (such as M and U) can easily be $200 more than the lowest ones offered on AC (K, L, T). Personally, I don't think it's a good use of cash to book a higher class to save on credits.