The Product Catalog exposes all of the available products and their configurations which are available for you to sell through WHCC. Additionally, you'll be able to identify the shipping options available to you and your customers.
You should only need to pull down a copy of the catalog as you add or change products, not as part of the flow to place an order.
This catalog will allow you to properly create a request to import into the Order Submit API for WHCC to producess and produce.
Fetch the Product Catalog to find the products and attributes to order