Creates a product discount and returns it.

• value_type is optional. If you leave it out, BillPro sets it from the field you send: amount makes an amount_type discount, percentage a percentage_type one. Send only one of the two.
• A discount comes off the line before tax is taken out, so it lowers the tax as well as the total.
• On recurring orders, the product's discount_for_initial_payment_only setting decides whether the discount applies to the first payment only.