If you order a big amount and falsely declare the value of the items and you are caught you can face jail of up to 7 years and fines of up to $500 000.
I have placed numerous orders to an overseas supplier and the tax that I have had to pay has been extremely limited. On a $10000+ value order I had to pay about $50, for example. This only applies to the U.S. and it also depends on what goods you are importing.
Other countries, includng most European countries, have to pay a VAT (value added tax) which is usually about 17.5% of the value of the goods plus the cost of shipping.