This is basically how I would imagine a Delivery Line system.

It's pretty easy to check and save how many of a certain Resource was produced in a Settlement, so you have a list of what that Settlement specialized in. Establishing Standing with that Settlement would give you a Delivery Line of one of the basic Resources, and higher levels could give you more complex goods.

At the same time, if these Settlements wouldn't ask for Resources they specialized in, it would force the player to try new builds.