Back from our trip last week, and I have some new info on this:
I shipped our bear spray to our hotel in Columbia Falls via USPS ground shipping, no problem. It was there and waiting for us when we arrived.
However, at the end of our trip, the post offices in both Whitefish and Columbia Falls would not let me ship my bear spray home. They said it's illegal, even shipping it ground, and they said that even though I've shipped it in the past from the Whitefish post office (and other USPS offices in MT and WY over the years), they said that this isn't a new policy, and no USPS post offices should have ever let me ship it in the past either.
I've always told the post offices exactly what it was, and that it needed to ship ground, and it's never been a problem to ship my bear spray before. But it's pretty clear that Whitefish and Columbia Falls absolutely will not let you do it anymore.
So at the airport before my flight home, I stopped at the Glacier Outfitters store (next to the rental car desks) and asked if they would take my 3 cans of Counter Assault. They said yes, they'll donate the cans to the Boy Scouts. Done.
Glacier Outfitters has a store right next to the backcountry office in Apgar. They rent cans of UDAP for $28 for a 3-7 window. You can drop the rental cans back off at Apgar, or as mentioned above they also have store and a drop box at the Glacier airport next to the rental car desks. So you have options and it's convenient to drop off your rental cans at the end of your trip.
Glacier Outfitters also sells cans of IsoPro stove fuel in Apgar. $7 for the large cans or $5 for the small cans, so pretty reasonable for inside the park. You can also get the cans at many other places around Glacier.