Oh yeah, now I remember. Shop signs were removed in the newer versions on EconomyShop. You can use /shop to set a block as a shop. Its usage on the plugin.yml: /shop <create|remove|list> [item[:damage]] [amount] [price] [side] For example: If I do /shop create 357 16 25 and then touch the block or slab that I want to make a shop on, it will create a shop selling 16 item 357's(cookies) for $25. You can use [side] if you want to show an item on top/left/right/bottom of the block (if you don't enter a side, it'll be top by default).
I remember seeing the plugin developer saying "I'm planning to add signs back" somewhere, let's just be patient and see.