I would recomend digital signing because it's a sure-fire legally binding way of establishing agreement. Adobe Echosign is a site that let's you send out contracts to sign digitally. They charge $15/ month but they let you send like 10 or so for free each month. Send the contract and Paypal invoice or whatever your payment processor is, at the same time. Once you get the money, then send the mp3. After that it's just a matter of them signing the contract and sending it back, which Adobe echosign handles for you.