I was going to say something about contracts, where an agent would gain access to a service but with some commitments. e.g. after buying a monthly bus ticket, the agent has a 'sunken cost' that it has to consider when evaluating other forms of transport. So if a better option pops up, they likely won't change at least until their monthly ticket expires.
The rational thing to do would be to choose the most cost-effective transport. The bus is free but takes longer (waiting/hopping buses/frequent stops).
profit = time spent working * wage rate - money spent on transport
For bus, money spent on transport is a fixed cost. For car,
money spent on transport = cost of fuel * fuel consumption rate * path length
If the cost of fuel, or consumption rate is low, or the path short, cars will be more cost-effective. If a bus pass is already purchased, it's cost = 0 (sunken cost as you said).
But if a bus takes significantly longer, it may be less profitable even if free. The choice must be made before the bus pass is purchased to gain the most benefit.
You could use these kinds of contracts to build hierarchies to solve the farm issue too. E.g. a field can be worked to produce food, but it's owner has specified that that workers can only deliver the food to a specific silo (also belonging to the owner).
I'll add an option to specify storage capacity of different buildings. If 0, they'll move it to a suitable owned building.
And rules for which buildings must be near each other (silo next to field).
As with the bus ticket example, the farm owner might sign a distribution deal with a particular retailer for some time period, where breaking the contract means he has to pay a penalty fee. The retailer may sign a contract with a particular trucking group to move food from the silos to the retailer, etc, etc..
So I would add an option to specify which outputs and which inputs require a distribution deal. Rather, which inputs/outputs are wholesale and which retail (end consumer).
I don't know why a deal with one retailer should prevent them from having a deal with another. PlayStation ships at Futureshop and Best Buy after all.