Limits for cargo ship loading

When trading a high value good for a low value good, even 8 long ships don't need a full module of the valuable good to come back full. For example, 7 modules of iron ore costs 57/750 servers. Unfortunately, there's no way to automatically limit deliveries to ships based on how full they are, so without manual control a tremendous pile of resources (750 servers) will be sitting uselessly in a ship.
(The dock module has a similar issue)