Stock Management Is An Art - Not Simply A Science
The more effectively you can fill orders the more money you get to keep from each sale. If you offer an extra $10 item you might only get to keep $1 or $2 after you end up paying for the item, storage facilities, incomes, and so on. But if you can discover a method to do something for $10 less than you're doing it now you get to keep the whole $10.