You can create an invoice disclaimer for a particular customer. Remember that generic disclaimers can be set up per company. Read Invoice Custom Information for further information.
Instructions
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Go to Setup > Customers
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Search for the customer account, then go under the Actions column and select Edit.
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The "Customer Information" window will pop-up.
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Under the Setup tab, click on the link: "Advanced".
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Click on Add disclaimer and enter the text you want to appear in all the invoices for the customer.