For one of the CA's that I sent out a letter to on Friday I looked up a physical mailing address for them and sent the letter there because on the letter they sent me only had a PO box and I know that nobody can sign (CRRR) at the PO box, right?
I have had no problems sending crrr to po box. they have to come to po to pick up the mail anyway, right? so i have not had a problem with this myself. all of my letters sent to ca with po boxes have been signed for.