How much does it cost to send mail FROM mexico.?

The ONLY place to buy stamps is at the local post office. Anything you send to the U.S. should be sent "registrado"...has a better chance of getting there. Costa about $3.00USD and takes 3 to 5 weeks unless you are in a tourist town. You would probably get home before your postcard got there. If you are at a resort...ask at the desk, they may be able to help you...but my experience here is that stamps are closely controlled by the post office.