money order

n : a written order for the payment of a sum to a named individual; obtainable and payable at a post office


Basically you take $300 to the post office, turn it into a money order, they charge a nominal fee, and you mail it to someone. That person exchanges it for $300 at a post office (or bank that accepts them). Since only the name you sent it to can cash it, it is similar to a cheque.