I'm stetting up a website for a customer in Missouri (I'm in Arizona). When I set up her PayPal View Cart/Checkout, I did not add sales tax. She wants me to add the 4.23% sales tax for Missouri to the Add to Cart. I told her that folks buying from out of state don't pay sales tax...only if they are in Missouri and making a purchase.
Now on my personal website, I have the sales tax field zero filled. If someone from out of state makes a purchase, there is no sales tax. If someone from within Arizona makes a purchase, the sales tax is added in when they check out.