Most design jobs do not lose money on the original brief. They lose it on the fifth "tiny" request that never gets priced.

Before you open the file, ask one question:

Does this change the agreed deliverable, or does it correct something already in scope?

• Correction of what you already promised → revision. Do it.

• New page, new feature, new round of "while you're in there" → new work. Pause. Price it. Get a yes in writing.

The sentence:

"Happy to do this. It sits outside the original deliverables, so I will send a short change request with the added time and fee before I start."

No apology. No essay. If they need it, they approve it. If they do not, your margin stays intact.

That is the whole system. The playbook is the checklist, the scripts, and the workflow around that one line.