This isn't something that happens often, but when it does, it's really a pain.
When you ship goods (like that camera or lens) with a value of more than $2500 from the U.S. to any country other than Canada, you must file a report of the transaction with the Census Bureau, which then issues a transaction number which mist be on the package. If you fail to do so, as I recently did, you risk the package being stopped and returned to you. When you ship by Post, this can take a week.
When I get the package back, in the next few days, I'm planning to use FedEx to resend it. My lessons from this: don't do these transactions at all, but if you do, don't use the Post Office. (FedEx has a downloadable form you can fax to them, and for a small fee they will obtain the proper clearance. To get the clearance yourself would require that you establish an account with the Census Bureau, which requires that you first take a test!)