Only thing to add is to send the USPS M/O by Priority Mail to have a record of delivery, just in case a fraud investigation becomes necessary. Receiving post office will also have a record of person renting the box, so you should be OK (but can't hurt to be cautious!)If you do send a Money Order, make it a Postal Money Order. If things go south, you now have mail Fraud and FBI has to investigate all mail fraud. Im with the others here, NO money ever sent to a PO box. That just sends up a red flag.
If your a bit uneasy about the whole thing, request that the rifle be sent from an FFL when shipped. It may be an extra $25 or $30 on your end but its a nice security. He will have to turn the gun over to a shop and let them deal with the transfer.
Oh yes...and ALL FFLs will require a copy of D/L for their record book.