What did you purchase? Where was it purchased? Most of the time US Customs hangs on to things just to clear it, ensure it's safe to bring into the US and also carries no pests / bacteria / viruses / etc on it. After a few days it should be released from customs and continue it's journey to your mailbox.
I had the same thing happen when I ordered from Cloud9Vaping.UK - my
kayfun(s) got held up in customs for over a week.