Here is a sentence from the documentation, because it is the whole module. "Once the setup runs you can review and update it before users start using the application. And once there are transactions, these setups cannot be changed."
That is a one-line note. It sits in the middle of a setup page, in the same typeface as everything around it, and most people read past it.
What "cannot be changed" means in practice. The cost method, the valuation structure and the cost profile assignment determine how every transaction was valued. Changing them retrospectively would mean revaluing history, which is not a supported operation, and would be an accounting event even if it were. Both halves matter: the system will not do it, and if it would, your client's auditors would want to know.
The recovery path if you get it wrong: a new cost organization or a new environment, and a data migration. On a live plant, that means a period end, a full revaluation exercise, and a conversation with the auditors.
Do not let anyone transact in a new organization until the costing design is signed. Not "mostly agreed". Signed. By Finance. In writing. That is a project management instruction rather than a technical one — which is why it gets skipped.
And the trap. Quick setup builds all of this in about ninety seconds. It is genuinely useful and it is genuinely dangerous, because it lets you create permanent structures before you have had the design conversation.
The build is under an hour. The design is a week — mostly spent in Finance's calendar. That last clause is the real estimate: the work is not hard, and getting the controller in a room, and getting a signature afterwards, is what takes the week.