cover letter's data in variable format

Hi all

i  made an object in abap which has functionality that after entering tcode , giving input like company code , invoice number and date , invoice form is generated (by using smartforms) also pdf is created to that form and finally mail goes to customer(by using document_send_api FM).

Now my client wants the which goes with attachment should have cover letter with variable data like month of which invoice generated, company name

should be updated. Can any one has the solution how to make cover letter including variable data.

Thanks

regards

Abhijeet