Customer Number in Invoice Mail Title

Hi Guys,

We have a solution in place that allows us to email customer invoices. The title/subject of the email is in format:

Company Name Invoice &VBRK-VBELN&

I was hoping that by replacing VBELN with KUNAG, we will be able to output customer number instead of the invoice number. However, this doesn't seem to work. What am I missing?

Also, if I need to output customer number and invoice number in mail title, how should the document title be defined in NACE?

Thanks for your time!