Actually, they do not make a commission until you actually sail, not at the time of the booking.
And as others have said, you are responsible for keeping track of your due dates. It would be nice if the TA sent a reminder, but it is not something they have to do. Do you get a reminder for all your other bills?
And I would not think that this would be something that errors and omission insurance would cover for a TA. It is not an error, and although I guess you could argue that it is an omission, I would not think that it would be by definition one that would be covered.