I've bought 2 from forum members, 1 from a seller on another forum and a fourth from DGW. All have shipped here to CA with no problems. I believe they all used USPS.
UPS may be fine with it so long as it's called a replica or antique. Legally, they're not firearms. Call it a rifle and they'll treat it like a rifle. Call it something else and you may get different treatment. Or not. Worst they'll tell you is no.
Looks like UPS recently started cracking down on some of the parts shipments for components that are illegal in some states & not in others. So calling them gun parts, especially if not serialized, could still be problematic.