In my case, I only need to charge residents of the state I live in (Maryland) sales tax. I don't need to charge anyone else. As I've yet to sell to anyone local, I haven't got myself sorted out yet with a sales license. Must get on that! My shop announcement and policies will show that Maryland residents have to pay tax and I'll issue a revised invoice via PayPal. I've seen other Maryland sellers do this.

I applied for a seller's permit in my state of California. On PayPal, there is a setting where you set your state sales tax. I guess if a customer buys from your store and PayPal sees that customer's address the same as your state, it will automatically add the state sales tax to the invoice. In my state, there are different sales tax for each county. I was told by my Board of Equalization that I should charge 9.25% for people in my county and 8.5% for the other CA counties. So I set 9.25% as my default CA state sales tax and will have to send an updated invoice to other CA county residents.

You don't charge sales tax to people who live outside your state.