This thread is a bit old, but it seems that the question regarding what tags or placeholders that can be used has not been answered.

What are the tags/placeholders that can be used in the email subject line?

So far the only answers I can see are:  Invoice Number [Invoice No.] and Company Name [Company Name].

Are there any others?  

I would like to know if there is a way to include the invoice date in the email subject line of the invoice.

Thank you.