shared business logic is it a model or should i place in controller

[eluser]Unknown[/eluser]
I have an application that has models for clients data invoices products etc
and the views etc

However there is also the business logic like get the client see if his invoice is overdue etc and do what it needs to do

I know I can do that in the controller however if I do that in one controller and if I have to do that
again in another controller I would have to repeat that code

so my question is should I have a model for those business rules or maybe a library

The same for say a payment gateway logic would I be best making that into a model as that may be called by more than one controller

thanks