Automatically having attachments in invoice emails

I would love a way to be able to automatically set a default attachment to send with every invoice I send out.  Like my terms and conditions.  I sell edible items that carry risk if not stored correctly and also can be damaged during transport.  My T&Cs are lengthy and provide me security.  I am currently manually adding these to EVERY email and I would love a way to automate that process (like when you create the template, can you have it added in there?).  I am sure other businesses have their own T&Cs for IP and other such legalities. 

ANYONE ELSE INTERESTED?