It is a bit different then method showed in previous post – this time its not transactional email sent at demand, but we will catch event fired after invoice is generated. So (I suppose you have your module already) in etc/config.xml add:

Then create your class at app/code/local/YourVendor/YourModule/Model/Observer.php (or use existing one) and add new method:

And invoice will be attached to email defined at:

Zrzut ekranu 2015-01-14 o 19.12.17

Seems to be complex, but its not that bad:

About $vars variable – in this one you can put everything which is set inside email template as variables.