If you want to send a prescription, test request or a medical certificate to a patient but only want to do so after the patient has paid for it, you can use the Attach Documents button, then add in the amount you'd like to bill and then send to patient.
Head on over to Billing
Click Attach Documents
Add the amount
Then click the Send button
The patient will receive the bill and only when they pay the bill will the document be released to them, all automatically done for you.