Because you are buying it in Florida. Look at it this way- your computer becomes a tiny little retail outlet for Brownells, Midsouth, Midway, or whoever. Your computer is in Florida. You wouldn't expect to pay Missouri taxes at a Florida Bass Pro because it's a Missouri-based company, would you? Maybe this seems a bit less significant to me because I'm in Montana- no sales tax

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In some states, there used to be a mechanism for getting a refund or exemption on out-of-state sales tax, but it was a huge hassle and raised red flags with the audit Gestapo. I don't know if there is still something like that or not.