Although I've had a PO box for nearly 30 years because of my LEO job, I never used it in any way for the FFL business.
No need to, as far as I can see.
As far as social media, if you're adept at using that, get your name out there.
Get your name on some general FFL search engines and also on the list of local FFL options that various gun websites like Buds maintain for their customers to pick from.
And while you're at it, so your best to avoid negative word of mouth from past or potential customers. That can sink you quicker than being on all the search engines and social media.
As far as working late, coordinating pickups, etc, get to know your UPS, FedEx and USPS delivery people. Get their contact info for your phone and ask if they'll accept texts or calls if you have some particular delivery issues that you need to coordinate with them. Figure out where your UPS and FedEx delivery hubs are to you and what their late pick up hours and locations are so you can go there to pick boxes up. You might even be able to have the boxes held at the hub for you to pick up during late pick up if you're routinely going to be missing deliveries at your licensed premises. That only works if you're a small FFL guy with an occasional delivery. The hub isn't going to hold 30 boxes a day for you if you're a full service brick and mortar store. Go to the FedEx and UPS websites and set up an account for shipment alerts, estimated delivery dates etc. Your customers are for sure tracking their boxes on-line ( some will call you literally as soon as their package is signed for, or if you've missed delivery they might call to chirp at you about that too ) and so should you. These are all things I do to track shipment and delivery of guns.
Oh, and since it's Christmas, don't forget your delivery person at Christmas. I don't give a damn if there's some corporate policy that says blah blah blah they can or can't do or accept something. They look out for me, I can at least look out for them once a year.
While you're at it, if you have other local gun dealers, get to know them. Network, share advice, help each other out. I am not afraid to refer people to my bigger competitors if I know I can't help them. They return the favor.