I may be mistaken. But for mailing a restricted firearm, the only ATT needed is for the seller-to-post office trip. Once the package is in the mail, it is a federal offense for anyone other than the recipient to tamper with it. As long as the recipient opens the package in the household that the RCMP has on file, the buyer shouldn't need an ATT.
For business/private sales in my experience, the RCMP issues a temporary ATT in conjunction with the registration transfer. This temporary document allows the buyer to transport the firearm from the seller's location ONCE.