Hi Tony,
Please follow this, this would not require ABAP intervention at all:
1. Go to NACE transaction, select required output type and double click on output type.
2. In General data tab, under 'Replacement of text symbols' section, enter program name to ‘SAPMV60A’ (for billing) and Form routine name as ‘TEXT_SYMBOL_REPLACE’
3. Now you need to add the mail title, To include a text in your Email subject line, you can either enter it in your Output Condition ('Mail title and texts'), or alternatively, in your Output Condition record, Cover Page text field. You can put &XVBRK-VBELN&- <Fixed text> as mail title
For mail body, you can use SO10 as suggested in previous posts. Please note you can use billing numbers dynamically in standard text also, just you need to include the variable (declared by ABAPer) in SO10 using DDIC (Data dictionary).
Please revert back in case of further information.
Regards
SD