I live 70 miles from the Canada border where we whine about Canadians crowding our Costco. I roughly counted their plates in one section of the parking lot and about every fifth plate was from Alberta. Edit: I have also witnessed Canadians in awe at how inexpensive was the stack of chewing tobacco they purchased.
Sin taxes vary across Canada and across the US. Even within the US the range is $6 to $12 [1] per pack of cigarettes.
In Canada [2] the range is $8.60 USD to $11.74 USD per pack.
Were they to declare on the way back as required by law, I strongly suspect it would have been cheaper to buy at home.
[1] https://worldpopulationreview.com/state-rankings/cigarette-p...
[2] https://www.statista.com/statistics/1307607/canada-regional-...