I called off my wedding and my ex's parents are trying to hold me accountable for a plane ticket for the honeymoon. Is this possible?
It may be possible. Right now, they are justifiable angry with you. However, there is a basis to say that in justifiable reliance on your promise to get married they bought plane tickets that they would otherwise not have purchased.
They have a claim for damages as a result. Whether or not they would actually go to the trouble of enforcing it is another matter.
Of course the reason for calling off the wedding could play a factor in determining liability.