You can simply download the file in it's complete form here. However, if you're a skeptic about random links like myself the fix is simple. 

1. Open GUIShop.cs

2. Find line 2262 then add the following: cmd.AddChatCommand(_config.scommand, this, CmdGUIShop);

3. Find line 422 & define your new command by adding the following: [JsonProperty("Sets Second Shop Command")]

                                                                                                                  Public string scommand = "s";

Assuming you referenced the code correctly you're good to go !