Gals/Guys ... I would strongly recommend that you review the US Customs Website BEFORE buying anything overseas (either online or within a store)!
CBP.gov - home page
So many people believe that when they order something from overseas, they are not required to pay customs/duties ... WRONG!!!
Generally speaking, a US Citizen can bring back up to $800 per person duty-free. However, some items are exempt (antiques over 100 years old) and some items cannot be brought in at all (endangered species, certain foods, etc.). When on the flight back home, you will be required to fill out the Customs form and identify all your purchases (make sure to save all receipts - the US Customs Agents could ask for them).
I have found that 100% HONESTY is always the best policy. Oftentimes, you will find that the US Customs agents will treat you much better and will many times assess you a lower percentage (resulting in a lower fee!!). Believe me, I buy a fair amount when overseas on holiday (> $5000) ... and still, my Customs/Duties have never exceeded $150. If you lie and get caught, you're life will be HELL from that point forward as they will *note* that you tried to bring in goods illegally and will SEARCH you every time you come back into the US regardless of whether it's business or personal travel.
Good luck ...