Listen to Bugweed on this stuff, he's got his facts straight. You can buy/sell within the state. NO MONEY, TRADE, or BARTER across state lines.
I also encourage anyone who is shipping Threatened/Endangered CP across state lines to include a letter specifying the plant is a gift.
Look, the truth is that NO ONE is going to open your package within the USA. The Feds just don't monitor domestic mail like that. Your letter, though, will be useful because the recipient will then have a receipt showing the legal source of the plant. I keep a separate binder with records of all Threatened/Endagnered shipments. Very useful.