There's no such option. The only attachment possible (which is automatic) is the PDF invoice, attached on the email.

To add room for another attachment would require custom coding work.

Custom coding work?
Would this not simply be an advancement of the product eg an update in a future release?
And if not then how much would it cost?
Thanks, Kyle.

it could be but we are not developing this or have this in our "to do" list for the next version. if you need it now it will have to be custom coded, that's what I meant.
we could of course add it in the future, but again, since we have no plans for it, I cannot garantee you it will be added, or when. To be honest we have not had requests about this before (that I can remember!), so even if it's very useful to you, it doesn't seem like a very demanded feature, that's why we don't have plans to add it...

to ask for cutom coding (and prices), please contact us at support@ joomlathat.com