Apologies if this has been asked previously.
Traveling JFK-SAN soon and have been unable to pre-pay for hold bags. Trying through MMB says

Your flight is ineligible for prepaid bags..

Same message on subsequent SAN-LAS flight. I’m sure we can pay at check in but how do I know the cost prior? Experiences with some airlines mean huge cost of booking hold bags at check in. Plus, if it claims pre-pay and save, why won’t it allow it?
thanks in advance.
For info we are UK citizens if that matters.