It would be nice if there was an option to auto-populate only non-craftable items (fireworks, xmas stuff etc) instead of adding every item to the config.

Would  it though?

my honest opinion is that there will be too many changes in the plugin for a feature that will not be used by many.. I suggest you adding the items into your personal config!

Inside the plugin GUIShop.cs code near the top, I added an exclude list. Simply add in all the item shortnames you wish to exclude from the config and generate a new config after saving/reloading. It's not ideal, but it will allow you to do what you want.