It depends. Amazon has changed a lot of things. So the individual states determine if there is a sales tax. Some states like Oregon do not have sales tax. The state then sets a percentage that is basically voted in. Then they counties and cities may add on to it based on voted in bills. So if we want public transportation, they pass a bill charging an extra .5% sales tax to help pay for it. Things like that. This is why, even though the tax is 7% in California, the county of Los Angeles has something like 9.5% sales tax.
Generally, the states can't make companies who are not located within the state pay sales tax. This has come under fire with Amazon being everywhere and nowhere. I think it was as of last year, Amazon agreed to start paying the sales tax. There was a long court battle. There is still no sales tax on digital products in California. Don't know about other states. So i don't pay tax on downloaded libraries. And if you buy from a company that is only located in another state, they don't have to charge you sales tax, but you are expected to include the amount of tax ( at the state rate of 7% because the state doesn't care about the county tax) you should have paid.
Does that answer your question?