Counterfeit check or money order / Western Union Scam.
Sends a fake check via Express Mail, Makes the check amount larger than agreed "by mistake", and asks to wire the difference via western union.
Do not deposit the check they send you! You deposit the check and it clears the next day, but after a few weeks you get a letter from your bank that the check was fake and you loose the money in your account.
Contact your local department and Western Union with receiver information.