If you are in California the DMV makes tens of millions of dollars a year selling all the data you give to the DMV, which is why I give them a P.O. Box.
Am I missing something? What do you mean the DMV makes tens of millions of dollars a year selling data to itself?
CADMV claims on their web site that they cannot accept a P.O. box as a residence address. I have yet to find anything in California state law supporting this policy, though IANAL. Their enforcement seems to be quite lax.