Internal cost accounting, activity-based costing, chargebacks to departments for Xerox photocopies, etc isn't what I'm talking about.

The CFO & acct dept does not send a literal invoice to the IT department where the CTO has his own mini accounting staff handle the invoice and cut a check back to the CFO. When corporate counsel reviews a Terms of Agreement for the IT dept's web service API, they don't send an invoice for $400/hour for "legal services". Not dealing with thousands of such "transactions" as explicit costs is one of the reasons firms exists (so the theory goes.) Internal operations do not send purchase orders to each other. They don't remit checks to each other. Each sub department doesn't have their own sales team haggling internal prices & discounts with other department heads. Etc, etc. It's the external companies' interactions that have all those costlier transactions.