From the product description:

Envelope X-RAY Spray ($16) turns opaque paper temporarily translucent, allowing the user to view the contents of an envelope without ever opening it. 30 seconds after the application, the envelope will return to its original state, leaving absolutely no markings, discoloration, or other indications of use.

Whoa – it’s the property tax thing. I’m not opening that.

Result.

awesomer