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In the US tax is calculated at checkout, as there is no general nation wide sales tax (or VAT) as in Europe. Sales tax varies by state, and by town, as it is usually composed of both state and local taxes. Prices listed are usually pre-tax prices, which is different from European practice of listing all inclusive prices (which in Europe is mandated by law anyway).
When buying something over the internet Sales Tax will be calculated based on where you live. On top of that Freight/Shipping(F/S) charges may be added.
When ordering something from abroad you will not have to pay the sales tax. But you may end up paying high shipping, and will most certainly be hit with a bill for VAT, import duties, and additional processing and admin fees.
However it is sometimes worthwhile to do this anyway. I have done this myself.
It is often useful to use a local forwarder. That is a service that gives you a mail address in the US, that you can receive parcels on. They will collect the parcel, inform you that something has arrived, and will wait your instructions.
Using such a service has a few advantages:
Just google "parcel forwarding" to find some. Do a bit of research first before settling on one.
When you tell the forwarder to ship your stuff to your home address he will fill in the necessary customer forms, and quote you how much the shipping will cost. Sometimes they will even give you an estimate of taxes can expect to pay afterwards.
I live in a place with a low VAT rate (7.7%), so the customs bill I get afterwards is usually low. However consider that if you live in a high VAT are (I believe it goes up to 25% in some parts of Europe) you may well end up with a customs bill of >30% of the value of your goods, on top of the forwarding costs. Buying that laptop in the US may end up not be that advantageous after all.