Just don't use a manila envelope with a metal clasp. Postal regulations considers that a "package".

As a side note, anyone sending items in a padded envelope without paying package rates runs the risk of the receiving post office charging postage due from the recipient. I know my post office here in NJ will as it's happened multiple times. They consider any padded envelope a "package" regardless of the thickness. Plus they consider anything in an envelope other than "paper" should be at package rates.