Hi Jamal,
Please use t-code FBZP and go to "Pmt methods in company code". Double click on the Payment method e.g. "C". This will take you in the screen. Click on "Form Data" button which will open its tree. Assigned your variant in correspondence and Line Items fields in frame named "Sorting of the".
Now run your payment program and see.....
Also I just noticed the Atul also replied with screen shot which will help you as well.
Regards!
Naseem