I wouldn't recommend a regular envelope. It may rip through the paper esp. if it goes through a sorting machine. And then your Arc would be orphaned until it found its way onto a postman's keychain. Good for him, bad for you.
I received AAA (in its package w/ batt & clip.) in a 9" X 7" bubble mailer. Postage was 83 cents, USPS first class from CA to RI. Without the extra stuff, it's less. (See below) And the envelope was probably < 50 cents.
I weighed these at home and got the postage rate from the calculator at USPS.gov
Weight of Light,ring, clip, package, battery, and mailer
=2.1 oz.= $0.83
Weight of Light, ring, and mailer only
=1.1 oz.= $0.60