Dear Ms. Murphy,
Thank you for posting your request on History Hub!
There are no special mailing instructions for the NATF Form 85 beyond those found on the form itself. You may mail it using any method you choose. Please consult with your local post office for questions regarding postage.
Please note that the National Archives and Records Administration (NARA) has suspended reproduction and digitization services until further notice due to COVID-19. Orders will not be serviced until operations can resume safely. Once operations resume, document reproduction requests will be filled in the order in which they were received. We apologize for any inconvenience.
Because of this, we would discourage you from spending extra money on any express or priority mailing options as this would not significantly speed up your response time. You may choose to send it certified mail, but this also would not affect when or how NARA would process your request.
We hope this is helpful. Best of luck with your family research!