For delivery confirmation, it must be over 3/4", unless it is sent priority mail. (there are certain exceptions that don't affect us much)

Here are some favorite links of mine:

The physical standards of first class letters, first class flats (big envelopes), and first class parcels are here:
http://pe.usps.com/text/dmm300/101.htm#wp1039555+

Delivery Confirmation here (includes that rigid box thing under 3/4" -- see section 9.2.2c):
http://pe.usps.com/text/dmm300/503.htm#wp1063871

Other good links:
http://www.usps.com/prices/first-class-mail-prices.htm
http://www.usps.com/prices/welcome.htm
http://pe.usps.com/text/dmm300/dmm300_landing.htm
http://pe.usps.com/text/dmm100/postage.htm

Robert

Robert