I was going to say, at the point they aren't letting you out the door, aren't they committing false imprisonment?

Besides, most places nowadays you have to explicitly ask for a receipt or press extra buttons within a 5-second time window to get one.

It depends on the reason why they're stopping you. If they actually think you are stealing, then no, they can stop you. https://en.wikipedia.org/wiki/Shopkeeper's_privilege