VM 3.0.12: how to apply discount only on product /not its custom fields/

[stawebnice]

Hi,

please, is there a way to apply a discount on a product only when it has string custom fields that increase a price?
We want to have a discount on rocking chairs, but not the pillows, and other accessories that can be added with them to the cart>

http://www.vintagelab15.com/design-verlichting/zitten/schommelstoelen/design-eames-rar-schommelstoel-transparant


1. Override
I don't know, if it is a bug or not, but when I set override price, custom fields which increase price stop working / they are added, but they do not increase price so it only shows price of product in cart

2. Calculation rule % discount
this applies discount also on custom fields which we do not want

3. Calculation rule - sum
unbelievable, but this actually increases total price...instead of  working as a discount