As long as there is more than one lawyer on the planet it won't be that simple.

Amazon has fulfillment centers in Texas and a subsidiary as well. This may be sufficient to trigger Texas' state sales tax laws.

These taxes aren't on the commerce in the items. Rather, they are written as taxes on using the items in the state. The courts (including the Supreme Court) have decided that this is OK under the interstate commerce clause as long as the tax doesn't make goods from out of state cost more than goods from in state.

So, if a state were to try to enforce a tax on mail-order goods from out of state, but did not tax those goods when bought in state, then they would be in for a Constitutional smack down.

This gets interesting when a tax was also collected by the state the goods came from. What the courts have decided is that the destination state can only collect the difference between what the receiver paid to the origin state and what the tax would have been for a local purchase. Thus, if you buy a car in a state with a 4% tax, and then take it to a state with a 6% tax and try to register it there, they can collect 2%.

None of this, though, is the issue in this case. No one seriously disputes that Texas can make Texas residents pay this tax. The issue is whether Texas can make Amazon collect the tax at time of purchase for Texas. If not, Texas will have to ask its residents to track their Amazon purchases themselves and remit the tax they owe. That's what most states do now--and it is widely ignored. Most people, in fact, have no idea that they are supposed to keep track of the out of state purchases and pay state tax on them.

The states get around this by insisting that companies doing business in their state must collect sales tax on state residents.