In accounting, inventory refers to goods or materials held for sale or production. What you are referring to is supplies on hand. Your bottles of polish are similar to boxes of pens or reams of printer paper. These are all supplies necessary for the delivery of services. What you are selling is the service. Such items are not figured into cost of goods sold, which are generally thought of as the cost of direct, identifiable parts and labor associated with an inventory item.
The trouble with trying to manage supplies through the accounting inventory function is that you have no way to record their use, unless you treat each job that uses polish as a manufacturing order with a bill of materials, and then invoice for the completed result of “manufacturing.” That seems like a very complicated and unusual way to run a service business. And it assumes that each job takes exactly the same amount of polish from a bottle. I doubt whether that is true. (By the way, when I referred to looking into the container, I was simply referring to making a physical assessment of how much you had left. I have no idea how big the containers are or how many jobs you can complete with one container.)
Going back to your original post, I recommend that you not use
Manufacturing Orders or
Inventory Items tabs. When you issue a sales invoice to a customer for a job, allocate the sale to an income account, named something like
Polishing Services. When you purchase the polish, use a purchase invoice if you are buying on credit or want to keep track of a supplier. Just Spend Money from a cash or bank account if you are paying immediately and don’t need to track what you buy from a supplier.
I encourage you not to make your business record keeping more complex than necessary.