The PayPal validation rules require that:

1. The total items cost must add up to the subtotal and,
2. The subtotal + shipping + tax add up to the total.

When using discounts, there is no place to set the discount so the transaction is rejected as the subtotal + shipping + tax does not equal the total order amount AND/OR the total of item costs will not equal the subtotal (depending on how/when the discount is factored).

We need a way to set discounts on transactions. This is a core feature I would have thought to have been included amongst the first implemented in the ReST API (common sense), but apparently not, therefore I seek another solution to it in the interim. I have scoured the internet and found no less than a half dozen other reports/complaints of this issue on StackOverflow and other sites.