There’s no reason to remove the CR1220 coin cell battery before shipping the board. You’re not able to ship loose batteries, or larger multi-cell batteries, but it’s okay to ship one watch battery if it’s installed in equipment, such as being inserted into the Mayfly’s battery holder. Don’t ship more than one at a time, and protect the Mayfly from rattling around, and protect it from short-circuiting on anything else inside the package. If you follow those steps, you are allowed to ship it via any US carrier without any certification or warning sticker requirements.
And as Sara said, the latest sketches we use (as found in the current ModularSensors examples) synchronizes the clock every time the board is restarted (assuming you’ve got a wifi or cellular connection), so manual setting of the RTC isn’t necessary. Or you can just load a DS3231 adjustment sketch, set your clock, then load your logging sketch. The CR1220 should last 4-5 years under ideal conditions.