Look at seller's disclaimer,

it usually states that 'xxx state resident add nn% sales tax', regardless of where pur purchased it, if you ask the seller to send to this 'xxx state', that's where the purchased is considered to have happened, so this 'nn% sale tax' should apply.

Stricly speaking, we are all supposed to pay sale tax on online transactions, but it's not enforced yet, which is why most people don't charge sales tax, but most businesses do charge sales tax for transactions that are completed in the same state.