Unfortunately, the Google book link in the previous post is way outdated (1974). The USPS POM 9 (refer to Philately Section) is probably a more up-to-date resource, even though it, too, is a few years old (updated 2008):
http://www.nalcbayarea.com/USPS%20M....%202008.pdfAs to the specific question as to what type of material stamps may be affixed to in order to receive a First Day of Issue postmark, I don't believe there is anything specifically in writing on that point. Of course, you are responsible for the postage and packaging to deliver and return the item if it is a form that is beyond that of a conventional envelope. There is a further provision that a 5-cent charge is assessed when more than 50 items are submitted for cancellation.