Syntax for pricing rule condition

I’d like to create a dynamic condition for my pricing rule.

The following does not work. Any hints are greatly appreciated.

(net_total - sum(item.amount for item in items if item.item_code == 'shipping')) > 100

Also tried:

(net_total - sum(doc.item.amount for item in items if doc.item.item_code == 'shipping')) > 100