Hi overwhelmed,
The best way is to get a letter from the CA confirming the PFD deal, and it's best to get it via mail. You want that written proof that the CA agreed to it, AND you have an envelope with their address on it so you have extra proof that they sent the agreement.
I've seen some posts from others who were able to do a PFD over the phone, BUT they requested a fax from the CA indicating that they agree to the PFD, AND they didn't pay the CA until they received the note in their hot little hands.
Whether you get a letter through the mail or by fax, you want them make sure that the agreement is written on paper containing the company's letterhead. The reason why you want this is if you received the agreement on a plain piece of paper, the company can always claim that they didn't write the agreement.