About the 3/4" thickness, if it is more than that, I think you must go via first class "parcel" rate or more expensive rates (like priority mail or parcel post, etc.).

It is all here, as I said in my post above:
http://pe.usps.com/text/dmm300/101.htm#wp1039555

"1.0 Physical Standards for Letters: ... ..... Not more than 11-1/2 inches long, or more than 6-1/8 inches high, or more than 1/4-inch thick. ... ...."

"2.0 Physical Standards for Flats : ... ... ... Not more than 15 inches long, or more than 12 inches high, or more than 3/4 inch thick. ... ... ....."

So, if it is over 3/4" thick, I think the best you can do is to use first class parcel rate.

Another nice page:
http://www.usps.com/prices/first-class-mail-prices.htm

Robert