If you are considering travelling to Montana from Calgary, one of the less travelled and quite scenic routes is through Cardston, Alberta and then the border crossing is at Carway, Alberta. The US side is called Piegan, Montana.
On the Canadian side there is a duty free shop called the Carway Gift & Coffee Shop. There you can purchase various duty free items. NOTE: there is No duty free coming back from the USA at this border crossing. So if you plan on coming back through here, you will need to purchase on the way down and KEEP YOUR RECEIPTS for presentation on the way back.
Carway Duty Free
Hours of operation: Daily 9 AM to 6 PM & during the summer on Fridays from 8 AM to 8 PM. If in doubt, it is strongly recommended you call them.
Below is a picture of the shop.
Our research shows this border crossing to be open for what is called Daytime service year round (7 AM to 11 PM)
The first gas station I can recall going through here is at Babb and I found it to be more expensive than the ones in St. Mary or Browning, but just by a bit.
You should also be aware that if you are thinking of using the "Going to the Sun" road there is a travel restriction as to the length of your vehicle. I drove up that road in a pickup and yes I would not want to do it in anything bigger. So if you are driving an RV or hauling a trailer, plan on using route 89 that goes around Glacier National Park.