This ability already exists. Inventory kits are independently priced from their components. If you sell Item A for X and Item B for Y, you do not have to sell the kit of A+B for X+Y. You can sell it for more or less.
So in your example of adding sauce and other ingredients to a kit of bread and cheese, just decide how much you want to add to the price of the kit to cover the non-inventory components.
@Deven, @Joe91 gave exactly the example I would have. Understand that it cannot be any more complex than this, because you have decided that salt and gas are not inventory items. Their purchase costs are posted to appropriate expense accounts, but Manager does not keep track of quantities on hand.
In a similar way, you could also advertise and sell cheese sandwiches for less than bread and cheese separately. You might do this hoping customers will come in because of your cheap cheese sandwiches, then buy a more profitable drink, too.