Since it's a lower receiver for a rifle, it's a long gun. You can ship long guns via USPS.
https://www.atf.gov/firearms/qa/may-nonlicensee-ship-firearm-through-us-postal-service#:~:text=The U.S. Postal Service recommends,used to ship a handgun.
Just put it in a non identifiable box and make sure you don't say what's in it and you will be fine.