USPS Priority insured w/electronic return receipt is how I ship all my C&Bs. Easier, faster, cheaper and more reliable than either UPS or FedEx IMNSHO. No special instructions or labeling required. You don't have to tell them it's a C&B, and you don't have to include an invoice - unless you want to or the buyer wants one included.
Fingers (Show Me MO smoke) McGee - AKA Man of Many Colts - Alter ego of Diabolical Ken; SASS Regulator 28564-L-TG; Rangemaster and stage writer extraordinaire; Frontiersman, Pistoleer, NRA Endowment Life, NMLRA, SAF, CCRKBA, STORM 327, SV115; Charter member, Central Ozarks Western Shooters
Cynic: A blackguard whose faulty vision see things as they are, not as they should be. Ambrose Bierce