The V1 API let you add custom discounts by supplying any field name at all with a negative money value. In the V2 API I can't get this to work at all, nor is it documented. Is it still possible to have custom discounts that aren't "Shipping discount"?
The weird thing is that, it should be possible to set negative values as described in the regular expression applied on the money value object used for item amount. Doing so, however, results in Paypal API error response as mentioned in the topic post.
P.S. This topic did not turn up when I used Paypal search function when I search for "discount" keyword. I unnecessarily created a new one before I found this.