paypal discount amount

OK
after a fair bit of poking about,
wpcommerce had this issue, so have ebay merchants, magento, prestashop, open cart, cube cart and others.

here is a quote

If you are trying to use discount_rate_cart or discount_amount_cart and are also passing an individual tax amount for each line item (e.g. tax_1), then the discounts are ignored by PayPal.

To get the cart wide discount applied you need to either pass no tax information at all or only pass the tax_cart parameter.

If you want to pass individual tax amounts then you have to use the discount_amount_x or discount_rate_x parameters for each line item.

Not particularly intuitive, and also not mentioned in the PayPal Payments Standard HTML variable reference....

best to strip out all extra variables, tax, discount_rate_cart and anything else your cart isnt using
DO NOT set values to zero or leave blank, they will cause any discounts to be ignored, regardless if they have nothing to do with the discounts.

given the amount of carts this problem has been reported on, Im seriously surprised that no one here has came across this problem

anyway, there's the fix (sort of)