HAHAH funny you should mention this, I had a package confiscated by customs a last year and it was a really small amount.
It came in a box which looked like it had been opened then retaped, then in another package which was also rewrapped, then when I opened it there was a note from US customs stating my package was taken by Customs and I would be contacted shortly. Well they called 1 time and left a message, I called back and left a message back but the never ever called again.
My buddy is a customs agent and he said that they do monitor some websites but mainly ones for narcotics or places that sell stuff to make certain drugs(especially GHB) but he said usually they look out for packages that are suspicious(packages that say cleaning supplies or odd things like bolts or tire tubes etc.)
Guess who's never ordering anything illegal online ever ever ever again?... ME!