Is a contract valid if it written on an envelope and involves stolen goods?

As a general matter, a contract can be written on anything and still be valid -- telegrams, emails, etc. are all perfectly valid media in which a contract can be memorialized. It is unclear what "involves stolen goods" means. For example, a contract could be perfectly lawful if its object was to hire a private detective to recover stolen goods. On the other hand, a contract to purchase or sell goods known to be stolen may not be valid.