there is no need for the shopkeepers to double the price of common goods.
Sure there is. My wife and I lived in Pullman, WA; about three hundred miles down wind of Mt St. Helens when it blew in 1980. Everything in that town comes in on a truck from somewhere. Interstate 90, the main road across the state was closed for a week by the ash fall. Trucks probably had to drive two or three hundred extra miles to get stuff into our area. That's extra gas and extra pay to the drivers. That has to get paid some how.
A few things were funny, we did actually come close to running out of toilet paper. Pullman is a college town, at the time about sixteen thousand students in a town of 25,000. St Helens blew on Sunday morning, by Monday afternoon the entire town was out of beer. On Tuesday a distributor brought in an eighteen wheeler loaded with cheap beer. I think he came in on back roads from Lewiston, ID to dodge state patrol closures on the highway. The price in the stores was then about twice what it had been on Saturday and they sold all of it in hours. A good lesson in the law of supply and demand in an emergency.