Okay, I'm with you now, thanks.
The category usage there is only used to decide where your shop will appear in the "shops" section when people browse through. The system prompts you to choose the cat & sub-cat that "best describes" what your shop sells. It does not limit you. Your *items* can be listed in the right, accurate categories as you wish. If you had multiple sub-cats that would imply your shop would be listed in multiple categories whenever anybody browsed through stores... which is an interesting concept. Once for ebid to think about perhaps.
As for are you in the right forum... it's as good a place as any.