Hi,
You need to use variables that will be filled with their values when creating the message. In NACE tcode, you can use those variables (they’re enclosed by &'s) in mail title and texts. Your title could be something like: My company invoice &VBRK-VBELN& customer name
In NACE, in tab General data for your output type, you’ll see a replacement of text symbols block with 2 entries: program and form routine. Here you establish the program that will be used to replace the variables with their values. Program for invoices is SAPMV60A and form is TEXT_SYMBOL_REPLACE (this will replace &VBRK-VBELN& by invoice number)
However, customer name is not a field available in table VBRK that means that you have to look
for the help of an abaper and create your own program and form (to replace the variables you need - invoice number and customer name-) and use them in program and form routine in replacement of text symbols in NACE, and finally, you'll have to adap your title with the 2 variables (enclose by &’s).
Regards,
JM