Shop Theme adjustments

I love the layout of the WP Shop Theme, however I don’t know how to post a product and have different prices for larger items of the same product. I don’t know how to do html. Does anyone know of a plugin I could use that would solve this problem easily, or would it be better to just go with another theme? You’ll understand when you see my site at http://www.franniedilley.com
Thanks so much!