Staying with the shotgun, but Haz is right. Small gauge.
In this scenario I see range as a "con".
Also, only 1 of the 4 has any stated experience with a fire arm, so even if AR's and hand guns were allowed, they would be more difficult to fire effectively even at close range.
This is a defensive situation. Everyone will be inside and should be behind any "cover that is available..counters, boxes, what ever.
Empty any window display items so that any looting will need to be done inside.
You might loose the big windows, but that is better than standing outside and trying to defend them.
Anyone comes in they die.
Large ammo capacity is not likely to be needed...this is not an assault, just looting..and they will be deterred when the first guy in is dead or bleeding out in the store front. You will likely have plenty of time to reload between attempts, if there is another attempt with, with dead bodies showing what they can expect.
And you don't want to risk hitting folks not trying to enter the shop, both for legal and moral reasons, so the less range the better.