With a recent update to PayPal's website (about 9/12/2018) it appears that they have broken the ability to use custom discounts with Express Checkount.
If you are creating a cart item with a negative value to indicate a custom discount/coupon the customer will receive an error after getting redirected to PayPal's website. When the customer logs in to PayPal they will get prompted with an error "We're Sorry. Things don't appear to be working at the moment. Try Again. Return to Merchant."
Unfortunately this error is not helpful as it is a catch all dialog for their errors and they provide no feedback back to the merchant to let them know that payments are failing.
Anyway, using Fiddler and watching the web traffic, I was able to see that an error was being thrown when their page calls the api https://www.paypal.com/webapps/hermes/api/shippingtax/charges. It would return a http/1.1 400 Bad Request error.
Obviously, if you change your store code to alter the value original item in the cart with the discount already applied it will not be a problem. But then you lose the ability to see the break down of the original price along with the discount price. However, this does not easily work for me as it will require a large code update on my companies custom store.
If PayPal was deprecating this ability I would assume they would fail the SetExpressCheckout() call, but that call succeeds with the negative cart item value. It only fails after the customer has been redirected to PayPal's site and after they logon.
At the time of this writing PayPal does not yet have a fix for this bug but I wanted to share the information to help others.
Edit: The problems is Negative Cart items along with using PayPal Instant Update. Yes, Instant Update was deprecated but PayPal States "PayPal continues to support existing merchants using this method, but please be advised new features and enhancements will not be applied to these integrations."
We have been a long standing merchant and have been using this feature for years and now it is broken.
