You can find this section in HostBill Admin → Settings → Products & Services → Your order page → Your product → Components → Sub-products
To add new sub-product navigate to the product/service you wish to add sub-product to and under Components → Sub-products click on Enable Sub-products. You will then be able to select packages you wish to offer as a sub-product (you can choose from already created products). You can select multiple products by holding shift/ctrl/apple button key.
If you have enabled sub-products in your product/service configuration and at some point you want to disable them in your order all you have to do is hold ctrl and click on the last selected item. It will deselect it. Also, this behavior is the same for all multi-option select fields.