Although it is sold as one way to detect a letter bomb placed in an envelope or similar package, most spy supply sales people will hint to you that you will be able to read what's inside of an envelope without leaving any indications of doing so.

One simply sprays the liquid on the envelope and the contents become visible through the envelope. Approximately ten seconds later, the liquid dries off leaving no trace.

Well the trick is that this transparent letter spray is nothing other than liquid freon. You can save yourself about $29 by going to a film retailer store an purchaseing some liquid freon. Pour the freon in an old spray bottle and use as necessary :')

A word of caution though: A lot of people get carried away when using the transparent letter spray for the first time and inhaling liquid freon over a period of time can cause nausea and related sicknesses. So be carefull and use in a well-ventilated area.