In general, for best results use a cardboard box. Boxes are cheaper -- free if you ask around at the right places -- and can't be folded or crammed into a mailbox.
Use padded mailing envelopes for floppy paper items or things of little value:
Use cardboard mailing boxes for rigid items or things of high value:
Here's a rule of thumb for envelopes: You've got something nicely packed in an envelope and ready to send off. Now suppose you drove your car across the envelope. Upon opening the envelope, would the items be unscathed? If so, go ahead with the envelope, otherwise use a box. Here's another test: bend your envelope slightly -- imagine rolling it into a cylinder. Would the spine of the booklet inside crack? If not, go ahead with the envelope, otherwise use a box.
Some have had success sending mini-figures in bubble-padded mailing envelopes. Cassette tape mailing boxes (around $0.50 at the U.S. Post Office) also work well for mini-figures and small LEGO® parts.
8 Jan 96 [TSL]