Before we get to solutions...
What is this highway tax? Is it the cost of transporting goods, the cost to recoup storing goods, or the cost of the price going down to get rid of the goods because they are taking up space? The only way I know of that a producer can "push" products out is by lowering the price until it is both, lower than the competitor's price, and lower than the costumer's "utility" price (I won't pay for something if I don't need it). I still don't understand what serviceexactly this tribute is paying for?
I would say "the cost of the price going down to get rid of the goods".
The service that is paying for is going from the producer to the consumer without the need of direct barter.
If the ware doesn't pay that tribute to money, it get stuck with barter. Money doesn't have anything to lose, it doesn't need to flow; the wares are the ones in a hurry to get to the consumer.