That's a good example (hidden hands). Feigning compliance, while actually preparing to defend. Certainly not something BG would expect. Would require training for accuracy. Works for appendix carry, not so much for 3 O'clock +.
DATD with a shotgun leveled at you seems (and probably is) suicidal. It did work in the Omaha Walgreen's robbery, but the shotty wasn't loaded. In this scene, it is.
Best hope seems to be when the BGs are distracted by the register. Don't think I would personally try the scoot out the door thing.
If you were aware enough as they came through the door, as mentioned, that would be your first opportunity. But again, you better be fast and you better be accurate...
If you were almost anywhere but at the register, you have distance (and hence) time to make your best shots.
Likelihood of BGs coming through the door, guns out and at the ready, seems slim. Most vids seem to show BG ambling about, pretending to consider purchases while "casing;" then puling up his shirt at the register to draw and point/wave/flail firearm demanding register contents.