I think I found the answer. In short, the person that owns the cart can't issue a coupon. PayPal Help: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_ECCustomizing Note: The ‘Enter gift certificate, reward, or discount’ link enables the buyer to redeem certificates, rewards, or discounts that PayPal issues. The link does not enable the buyer to redeem incentives that you issue.
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