Client won't care about terrain. Probably need to just charge by distance, plus a pickup fee. Say $5-10 plus 50 cents per mile? Just call around to a car-based courier and find out what they charge. Probably .65/mile I'd guess.
Then call a local law firm, they are most likely to use a courier. Ask how many deliveries they have and what they use now, then ask if they are interested in a lower cost alternative.
It will be tough, and don't expect to get rich. But maybe you can afford to eat at least.