Writing specification: can business rules apply to a specific vendor or third party service implementation?

I am writing a simple specification for a module that is part of a bigger call center application. First I write down business rules, then application logic. I realize that some rules apply only to a specific third party telecommunication service API that we are using, like

There are three invoice templates in the telecommunication service backend. If template 1 was chosen, send copy of invoice to xy@example.com

It sounds like a business rule, but it refers to a specific implementation – other service providers might have a different templating system etc. Choosing another service API would therefore mean to update or change these business rules, which feels wrong.

Is it then better to understand this as application logic?