Weirdly for me: IKEA. I’m within ~240 miles of an IKEA in Canada and an IKEA in the US.
While they’ve started to inflate some items to meet currency conversion rates, some items are still cheaper for me to purchase in Canada directly and bring back to the US.
For instance, even at small scale: one BILLY bookcase, article number 205.220.46, is $90 CAD (~$65.70 US) at IKEA CA and $79 USD at IKEA US.
YMMV coming back across the border but in my experience I just got waived through the border every time I told them I was “just coming back with some cheap crap from IKEA”.
Is your time and car free or do you want to make the trip anyways?
Car's 11 cents a mile, that's less than twenty bucks in gas, me spending ~5 to 6 hours total back and forth retrieving it is still worth so much more than waiting days on end for freight shipment (and the hundreds that can cost, combined with the messy scheduling commitment if you buy any large goods -- I just checked, it's $289! for that Billy to be shipped to my doorstep).
I have in fact brought a rolled up full size mattress home in the back seat of a Fusion Hybrid (it fit! with room for other things!) and it was a great cost savings. As a bonus at the time there was an additional sale in IKEA CA on the mattress that US didn't have, so I saved even more.
Travelling to a no sales tax state for large purchases. Sales tax is roughly 10%, state with no sales tax is 150 miles away for me.
Doing the math, 300 miles round trip, 30 miles per gallon, $4/gallon for gas, if I'm buying something that costs more than $400 I get a free trip to other state.
Downside is that you're only breaking even for the time, but if you're making a $1,000+ purchase then it's definitely worth the time for me to make the trip.
Some states have that as a "you should/need to declare that as a use tax."
https://en.wikipedia.org/wiki/Use_tax
It's likely poorly enforced, but it's on the books and it's a complicated one to track. It was more of a concern when internet sales didn't collect state sales.
There's also Simplified Sellers Use Tax lawsuit that was recently in the courts.