I work for a company (In Canada) that regularly ships boxes weighing just over 5 pounds. Within Canada it is most economical to use Canada Post.
But if I am shipping 40 pounds, it is more economical to ship via a regional courier, not one of the big national/international ones.
OP you said you do not have a Greyhound office close by, do you have another bus line that services your area? For one of our customers who lives in a fairly remote community, serviced by a local bus line, I can still send via Greyhound. There is an agreement between the bus lines to carry each other's parcels.
Sometimes you need to think outside the box, no pun intended. Who is making deliveries to the places you need to send things?
I work in an industrial area, several of the truckers make trips to Vancouver a couple times a week. They have been known to bring back passengers once in a while.
The company I work for has delivery trucks for our own products. The odd time the drivers carry something extra. It is not an every day occurrence and is dependent on there being room on the truck.
Many companies have internal mail services, sometimes you can piggy back on that too. Once again my company will use the credit unions internal mail service instead of us paying the cost of shipping to another credit union location.