how to implement Voucher payment with paypal

I have an asp.net mvc e-commerce application that requires voucher payment. how to pass the voucher amount/ How to set the discount amount on the entire cart? - to paypal t开发者_如何学Chat will be deducted from the total payable amount upon payment?

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Just check the voucher on your own site, subtract the value, and only send the remaining amount to paypal.

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I've found the answer! the variable name is

discount_amount_cart

thank you @GolezTrol