Product Not In Product List
You cannot sell products in Checkout unless they are in your Product List.
To handle the sale of products that are not in the Product List, create a new product that has a name and code of NonStock, and make sure that there is no check mark in This product has stock. If you will be selling taxable and nontaxable products, create a second product that has a T at the end of the name and code to indicate that it is taxable and select its tax rate.
When you add this product to a quote or order, you can edit both the product's name and price (Manager login or Editing Price/Name Preference required for employees).