If you live/buy in ND you pay ND sales tax. . . regardless of amount.
If you live in ND and buy from MT, SD, MN, etc. you could previously avoid the tax due to a purchase across state lines. The way I am understanding the current situation the only people that do not need to worry about the tax are those living in tax free states. Even if you live in ND and order from MT (no state sales tax?) you still have to pay the ND sales tax to the seller in MT -- who then has to send it back to the correct govt. entity back in ND.
Not sure if that makes sense (or if 100% correct -- but that is how I am interpreting from previous posts). Luckily as a Veterinary Clinic I have not had to collect sales tax on services. . . but they are trying to change that.