add discount to purchase unit

[abdelwahid elmeghit]

Description

-The total order amount with an optional breakdown that provides details, such as the total item amount, total tax amount, shipping, handling, insurance, and discounts.

If you specify the amount breakdown, the amount equals item_totalminus discount.

• You can reference the official documentation https://developer.paypal.com/docs/api/orders/v2/#orders