Economists consider the next unit when making decisions. For example, suppose I want to eat another bite of spaghetti. I would think, is the benefit greater than the cost? If it benefit is greater, I would take another bite of the Italian pasta.
What if I'm a Logistics manager and I want to start my own distribution center. Let's assume that the costs for delivering the inventory to the retail outlet are 5% higher for me than if I would use the local intermediary. Let's also assume that my total costs would be lower because I would have more efficient delivery, higher visibility over stock, less damage to my inventory, quicker reorder time, and a better handle of changing consumer tastes. So my total costs decrease.
This is what Walmart* found when they centralized their distribution center. They relied on total cost where economics would have told them that the marginal cost was greater than the marginal benefit.
I am going to end now as the sum of the costs of my marginal thoughts outweigh the sum of the marginal benefits.