It is never a good idea to send multiple tax returns and payments in the same envelope --unless you enjoy waiting 2-3 ties as long for your returns to be accepted and processed.
Always put only ONE return and its supporting papers and payment in one envelope.
When mailing a printed return it is always a good idea to go to the Post Office and mail it 'Certified' with 'Return Receipt'. That way you will have proof that they received it and when it was received.
