When I got to day 13, everyone would come and leave INSTANTLY. They would just get their order out like "Get me a polearm please." and then they would leave. I had to average out what everyone wanted and make it before they came in order to make any money. If anyone know's how to fix this, please let me know.

I am having the exact same problem

Ran into the same issue and noticed that some customers wold walk in and demand an item then walk back outside but hang out by the door.

Brought the item they wanted to them (should have charged for delivery) and got the sale.