A few things come to mind…
You can send a custom extra within the order to cover the additional work, or advise your buyer to place another order with the new requirements. It might be useful to list exactly what the buyer purchased and what his requirements were, what you delivered, and how is the new work outside of the scope of the agreed requirements.
You can also politely remind the buyer that they have checked the little box stating that all the information they gave you is accurate and complete, and that asking you to do more might cost extra.
Another option (or something to combine with the above), is to point out Fiverr;s Terms of Service: "Requesting to gain more services from Sellers beyond the agreed requirements by using the Request Revisions button is not allowed.
Fiverr Customer Support will cancel orders based on, but not limited to, the following reasons:
- Buyers who abuse the Request Revisions button to gain more services from Sellers beyond the agreed requirements.
- Buyers who threaten to leave a damaging review to gain more services from the seller not related to the agreed requirements."
Unfortunately, the buyer might decide to demand a cancellation or to leave you a bad review if you do any of the above.