While it is not illegal to send cash in the mail, it is unwise. If cash is lost, there is no recourse for either party.
The fact that you have posted your mailing address publicly has alerted would-be thieves to the possibility for a bonus at mail delivery time.
Better to send a Postal Money Order; convenient too, as both parties will be visiting the post office at some point during the transaction.
(Yep, I'm no fun at parties either.)
