The sales tax is not applicable on purchases that span multiple states. It doesn't matter how hard you argue that one state or the other
should have the ability to tax a transaction, whenever there are multiple states in the equation, none of them are constitutionally able to do so. If there's an endpoint in one state, and an endpoint in another state, it crossed a border somewhere and earned the "interstate" adjective.
The state only has the power to tax intrastate commerce, where both buyer and seller are standing within its borders. If you are loud enough and persistent enough, you can even convince a seller to not withhold sales tax on a purchase where you are physically present, by making a solemnized claim that you intend to immediately export the goods you are buying out of the state. Businesses do this all the time. (But then they pay use tax for their own state, because it's cheaper and more certain than arguing their case all the way to SCotUS.)
The tax is an excise on the buyer. Sellers subject to the state's jurisdiction must collect it, by law, on behalf of the state, whenever they are able to do so. It is an undue burden on sellers to be forced to operate as tax-collection agents for all of the states, including those they have not directly chosen to do business in, by establishing some kind of persistent operation there. There are hundreds of individual jurisdictions able to levy sales taxes on the domiciled residents of their state making purchases within the borders of their jurisdiction. What a towering barrier to entry it would be if, in order to operate so much as a catalog mail-order lemonade stand, a business had to purchase software licenses or tax-handling services able to resolve taxes for all the states and territories, and all of their tax districts, and possibly also an alternate rate within that district.