As an FFL myself, I can see both sides of the equation. I'm home-based, so the "upsell" option is near zero for me, but let's pretend that it does exist. So, let's go....
You are an internet gun buyer. Why? Because the prices are pretty damn good, even when you factor a transfer fee and Use Tax. The typical response here to a high transfer fee is "what a dumb shop. They should charge a low fee and try to sell the transferee some ammo, mags, accessories, etc. and make their money that way." Here's the flaw in that logic: INTERNET buyer! If someone is willing to go through the extra hassle of buying a gun online to save some money over buying local, do you really think they are going to pay "local" prices for accessories, PLUS Sales Tax? Really?
That internet gun buyer is also buying all their accessories and ammo online too. Even with shipping, it's highly likely that shipping is going to be less than the 9.5% sales tax (soon to be 10.0%+) So where is the upsell opportunity in this scenario? You are already dealing with a "bottom dollar" internet gun buyer, do you really think they're going to be a retail-dollar accessory shopper? I highly doubt it. They are walking in to get their transfer, then scoffing at the "outrageous" cost of ammo, mags, etc. in that shop.
Now, here is how I approach the scenario. I can't really upsell products for sale at the same time (don't keep any inventory on hand), but I look at it as an opportunity to build a relationship with the customer for future business. "What are you looking for? Give me a call next time before buying and maybe I can get you a better deal!" I try to understand what their current needs are, as well as their future needs. My "upsell" is a future sale, maybe a handgun, rifle or even an NFA item. I have that luxury though, to get to know my customers on a more personal level.
Sometimes that works, but not always. I had one customer buy a handgun online for a "decent' price. After adding in Use Tax and my transfer fee ($35), he spent almost $2,000. I told him afterwards that I could have ordered that same exact handgun for him for $1,600 out the door. Yes, he would have paid $400 less, and I would have made $150 instead of $35. So we both lost out in that deal. Guess what, a few months later, another transfer shows up for him, and again I could have saved him $250 and tripled my profit. Another Win-Win that didn't happen. I told him again how much I could have saved him, and his reply was "yeah, I forgot. It was just so easy to punch in the order late one night."
The amount of time spent dealing with either transaction is nearly identical (handling a transfer or ordering in the gun.) But in one case, I make $35 and the other it could be $150. If you had the option to trade your time for $35 or $150 for the same exact work, which one are you picking?
One way of changing or discouraging that type of shopping? New guns are 10% fee plus Use Tax. The question is, does that change the behavior of the customer, or just send them down the road? I generally feel it results in the latter, which is why I don't personally run such a pricing scheme. And I don't have the customer base anyway to "lose a few" with such a policy. But a large retail store, looking at paying an employee $20/hour after benefits/taxes/etc, to make that same choice between $35 and $150? They've likely done the math and figured out that the transfer is actually losing them money, so losing a customer that is costing them money is no loss at all. Add in the additional unproductive and money-losing time dealing with the "showrooming" customers, and it might make even less fiscal sense.
Just some points and counter-points to consider.