I have heard rumors that postal employees in the sorting centers are destroying Trump ballots (presumably by checking the envelope with a strong light)—literally all of the sorting center postal workers I have known are big union guys and rabid ‘Crats. So unless you want your vote destroyed—you might add some dark/security paper around the ballot if they gave you a translucent/non-security envelope to send it back to the Board of Elections in your area.
If you can, walk-in the ballot directly to the county clerk and recorder. I do.