In addition to informing your client verbally and in writing that "final" proofing is their responsibility, you could also state that minor edits found within a specified time would be done at no charge to them. However, errors found in the original material that need correction would be charged at the standard hourly rate.

You may also want to indicate that materials will be processed "as is" with no editing unless requested by the client and depending on the type of project you are working on.

BTW, I don't carry liability or E&O insurance.

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Mattie Stokes