Being able include a discount code for certain items will depend on the shopping cart system you are using. I am not sure that paypal currently offers that option but go to your paypal account > Merchant Services > button generator > add to cart type and look to see if a discount option is not available.
Otherwise, go to Insert > ecommerce > configurator and look at both Mal's Cart and Roman Cart. Both of these carts can be used with paypal as the payment gateway.
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