Could you switch to constraint_methods for the billing fields? I haven't done this, so I'm not going to give an example. But look in the perldoc for Data::FormValidator in the section called VALIDATING INPUT BASED ON MULTIPLE FIELDS. It refers to the perldoc for Data::FormValidator::Constraints and its section WRITING YOUR OWN VALIDATION ROUTINES.

I think those sections and a bit of play will work.

Essentially, you write your own validation routines for your billing address fields. Each of those routines can resceive the value of the checkbox as a parameter.

Phil