If your license is in the mail, then you already have a PAL number, which you can obtain by calling CFC 1800-731-4000, talk to an operator in licensing.
Then you can call Frontier, give them your PAL number and other info, then they can ship/mail you the rifle. If you are in the store to pick-up the rifle, they may or may not ask to see your PAL and DL to confirm if you are who you say you are, depending on whether they know you or not.
If buying from an individual, you can do an internet application and usually gets approved while you are on-line, and still on the phone talking to the operator, who will be on the same screen as you. She can walk you through the process. Did this on a recent transfer and immediately got my New Reg. Cert. No. on the printout. She gave me a Reference No. as well.