We're planning a trip in late June that potentially involves flying into Calgary and renting a car to visit Banff NP and Glacier NP, then driving down to Denver and flying home from there. Is it possible without it being prohibitively expensive to do a one-way rental across international borders?

Would it be less expensive to do one rental for the Canada portion, drop off in Banff, then rent for a day to cross the border and drop off in Kalispell, then do yet another rental for the US portion of the trip?

Are there other options other than flying for crossing the border without a rental car in that area?

There will be four of us traveling together, all US citizens. Any advice would be greatly appreciated.

Welcome!

With a very few exceptions, it is generally nearly impossible to rent a car in one country and return it in another. It's fine to drive into another country, but rental car companies don't want to the hassle or expense of having to return cars across borders in most areas. In the few areas where you might be able to do so (i.e. Vancouver/Seattle), there will be a huge one-way fee. Otherwise, it simply isn't permitted.

Calgary is a major rental area, but Kalispell is not at all. Unfortunately, there are few to no other transport options - there are no cross-border buses or trains in that area either. Just about everything runs via Vancouver or western BC.

My strong suggestion would be to rent & return your car for Canada in Calgary, then fly to the US and rent another car there. Or arrange your schedule (I hope you intend to visit Japser NP park as well if you are coming all this way) so you can fly into Kalispell or other Montana airport, rent the car there, then drive to Canada, then back to the US.

I also hope you've booked your accommodation in Canada - late June could be a very tricky time to go to Banff NP if you haven't made reservations already for accommodation & rentals. July 1 is a national holiday here, and often one of the busiest weekends of a summer that is already shaping up to see record numbers of visitors. People have been reporting increasing issues with finding accommodations, particularly on weekends/holidays in the Rockies.

Where would you be flying from? I did once combine a trip to the Canadian Rockies with a visit to Glacier NP.
I/we used a train option taking the ViaRail Canadian to Jasper and spending 2-3 days and then back to Vancouver on the train with a bus to Seattle.
We then took the Amtrak Empire Builder to Whitefish where we rented a car for Glacier. From Whitefish we took the Empire Builder all the way to Chicago.
After a couple days in Chicago, we took the California Zephyr back west through Denver to Glenwood Springs.
Another airport to consider would be Spokane WA depending on where you are coming from.

I posted on this exact subject a few weeks ago. I generally got the same responses. I called Budget, and checked Hertz on line. Hertz made a quote that was somewhat high, so I called Budget. They said that a typical Canadian car didn't go into the U.S., but if we gave them a little notice, they could reserve a car that apparently came from the U.S. However, a 2 day rental was close to a thousand dollars. It was high, but not as much as 4 people flying. I don't feel real comfortable about the situation, so I would call about the reservation, and get something in writing. Then you must let us know how it works out. Thanks

We are flying to Calgary, going to the Canadian Rockies, and then driving to GNP. I have rented a car in Calgary from Avis. Will return the car to Calgary. They were the only company I could find that would allow me to take the car into the US without a hassle and for a reasonable price.