You are limited to very few options for Powderhorn shuttles from Grand Valley. There have been several shuttle options either go out of business or discontinue service to Powderhorn, so please read this if you are looking to understand which ones are still in play this season.
American Spirit has been around for a little while. We know for certaint that they will bring you from your spot in Grand Valley up to Powderhorn. They don't, however, post their prices, so we're not sure what it costs.
Here is their website: American Spirit Shuttle
Here is their Phone #: 970-523-7662
There used to be a free bus service sponored by Powderhorn and some medical companies, and it was pretty great in its time. With pickup spots in Fruita, GJ, and Pallisade and leaving the mountain at 400pm, it worked pretty well for the people who used it. Unfornately, last season not many used it and those who did vandalized the seats or were jerks about leaving the mountain on time. So it was discontinued last season and may not be back again.
We are pretty sure this was a scam, but if you found this page when searching for shuttles, know that they are no longer in business--if they ever were. So don't spend any more time looking for them.