At my post office, one of the counter clerks revealed that they have been told to keep the "free" boxes behind the counter to promote sales of the "you buy 'em" boxes on the sales floor. Of course, eventually I just ordered them (free) from the post office because of the amount of stuff I ship.

Mike Heron is also right - if you are shipping less than 2 or 3 pounds, there are better ways to go. But chips are quite heavy and 200 are ideal for a flat rate shipment.

When I stand in line at the post office, I always discuss with other customers just what their needs are - especially when they reach for the express mail envelope or priority mail flat rate envelope to insert a letter or bill payment. Here's a good way to think of it:

Letter in plain envelope: 41 cents - takes 2 to 3 days
Letter in priority mail envelope: $4.60 - takes 2 to 3 days
Letter in any envelope that you sealed with free priority mail tape: $4.60, 2 to 3 days
Letter in express mail envelope: $16.25 - takes overnight if you mail it before the deadline (usually around 6 PM)
Letter in express mail envelope mailed after deadline: $16.25 - takes 2 days.