Just sold a guitar on reverb, boxed it and now i don't know what to do.

sold a guitar to a guy in Maryland, and I have boxed the guitar with a bunch of bubble wrap and news paper. Passed the shake test, and now I don't know what to do. I got a packing slip and that's all, I know o should take it to USPS, but how do I ship directly to him? should I get a shipping label from reverb and use that?