In the past , the rules at the border for bringing liquor into the US were pretty much ignored for small purchases. In my experience, 3 or 4 bottles was a small enough amount that the US Customs agent would just waive you through.
So, my wife was going to Canada for some Caesars Windsor offers and decided to purchase some Crown Royal. For October, the duty free shop was running a special, Four 1 liter bottles for $61 USD. In the US, a fifth of Crown Royal is about $23. 4 litres is 135 oz and 5fifths is 128 oz. 4 litres for $61 is a whole lot less than 5 fifths for $115. Its about half the price in Canada at the duty free.
My wife also received a $10 off her purchase coupon from the duty free shop. Sounds too good to be true. And it was. She pulls into the duty free shop and there is only one other car in the lot. After purchasing the 4 bottles, she goes to the customs booth and he tells her that she has to pull over , go inside and pay duty on the purchase. I have done this for at least 10 years and have never paid a duty. Her tab was $2.85 times 4, so about $11.50 and it took an hour. She also said that the waiting area had about 6 other people for the same thing.
So, either Canada is going after the relatively small duty now, or the US is tired of people buying liquor much cheaper in Canada or maybe someone got caught smuggling and using a duty free liquor purchase as cover. Regardless, it is now a longer, more expensive proposition to bring back liquor to the US.
If things stay like this, I predict that duty free shop with either greatly downsize or close up. When I used to go there, the majority of people were buying liquor.