Have to click on the link "list of Google Product Categories" and then find the category for your products. If you sold fish supplies and the product was an aquariums... look at the list and find the category that would include fish supplies. In this case, I would add the following:
3238 - Animals & Pet Supplies > Pet Supplies > Fish Supplies > Aquariums
... this helps tell Google what your products are about. Do this for each product and then Google will know what you sell. Hope you found this helpful, if so please click "like" below to let me know.