-have 'validate' subroutines defined (I<$options> values might
-be altered.) Returns error message, or empty string if valid.
+have 'validate' subroutines defined in the info hash,
+then validates the entire hashref if the price plan has
+its own 'validate' subroutine defined in the info hash
+(I<$options> values might be altered.)
+
+Returns error message, or empty string if valid.