How do I include a discount on a customer's monthly statement via Pastel?

[Spdy]

Good Day

How do I include a discount on a customer's monthly statement via Pastel?
(This will automatically discount the customer's/client's monthly statement
each month, before sending it out to them.)

Please be so kind as to indicate by means of a step by step process.

Thank you very much. Your assistance in this matter will greatly be
appreciated.

Kindest Regards
Melissa