Last time I checked, even JP won't ship their lowers into California. For you to ship the upper is no problem at all. If it was me, I'd get a box, remove the stock, mag release and pistol grip from the lower and put them in the box along with the upper and ship them directly to your customer (which is perfectly legal). It will probably be easier for you to find an FFL to deal with a "stripped" lower, and get it sent to an FFL in California rather than the whole rifle.