This is from the usps website:
"Yes, you can print a First-Class Mail® parcel label for packages weighing 13 ounces or less. Delivery Confirmation service with First-Class Mail is limited to parcels only. A parcel is defined as any piece that:
Has an address side with enough surface area to fit the delivery address, return address, postage, markings and endorsements, and special service label; and
Is in a box or, if not in a box, is more than 3/4-inch thick at its thickest point."
Direct link:
http://faq.usps.com/eCustomer/iq/usps/request.do?create=kb:USPSFAQ&view()=c%5Bc_usps4537%5D&varset(s...Scroll all the way to the bottom under other information.