I know J316 to be a shop that is often packed, and their prices are not the lowest, but to overcharge double is something I have not known the shop to do. I know they do make mistakes when the shop is crowded, as we all do make mistakes sometimes too... but maybe you can go back and talk to Samuel. He is a reasonable man and if you explain to him, I'm sure he can do something for you. I've been sending friends there and when my friends come back, I ask them how much they paid for the stuff, and everyone of them have not been overcharged.