missvee, that is basically how I look at it. I factor it all in. For me NAS isn't really a sale. Even if I manage to get my items without duty, which I often do, given current exchange rates, I'm still, essentially paying full price. (I.e the difference between the CAD and USD means I'm paying the equivalent of the US regular price for my so-called "sale" item.
But if I'm paying for an item I otherwise wouldn't be able to get, that I love, and getting a jump on my plans for the coming season, that makes it all worthwhile. Of course, living relatively near a border makes it easier again, since I can take items across to ship them back for free if they don't work out.
With online ordering generally, and especially from locales like the UK and the US where I may end up incurring delivery fees and/or return fees and/or extra duty -- if I feel extremely doubtful, I won't order. I rarely order from brands whose stuff I have never tried on in person at least once. I hesitated for two years to order from Boden for that reason. Once I bit that bullet, I was overall pleased with my choices and got a few good dresses out of the deal, but I still had to pay about $50 in return charges for stuff that hadn't turned out as I'd hoped. C'est la vie. I just added it to the cost of the (mostly sale) items I did keep. In that case, the dresses were still a bargain in CAD.
Fits are not necessarily consistent across a brand and fit models can change from year to year -- but at least if you have tried something by that company you have a general idea of size. If I plan to order online, I read reviews of the item and product descriptions really carefully. And I still sometimes strike out. I made a really stupid decision recently, giving in to the insistent FB ads of a small "independent" company that supposedly sold the best raincoats ever. TERRIBLE waste of my money and worse customer service -- I can't even return the thing. Oh well. Lesson learned.