Hi Nicolas,
thx for the tips.
FYI, it is not possible to have a negative price for products with the normal admin HS interface. It will be converted to positive (ABS function). So the only way to do that is by modifying directly in the database... not nice.
Also, it looks a bit weird to have a product in the cart like that because you can change the amount etc as end user.. so that was a dead end.
I finally took the approach of adding some Joomla User groups and creating different discounts which are only available for that specific product group and also only for a specific (newly created) Joomla group.
Then created a Plugin which will calculate the total amount for that specific group and if above a threshold (which can be set in the plugin) it will add the shopping user to the next Joomla group. That results in HS now applying the next discount.
If the total amount is reduced again, the user will be removed from that extra group again.
(I just needed two steps above the standard discount)
Plugin ended as being a little bit bigger then expected but it seems to work flawlessly..
Hope this helps others perhaps.