A spokesman for the ombudsman suggested that the letters come from a Kortrijk businessman who is trying to generate advertising revenue and is working together with another known fraudster.
The ombudsman and the Flemish small businesses organisation Unizo is urging people never to pay the invoice.
Ombudsman Bart Weekers: "The letters look professional. We believe it has been sent to many small businesses. They are being asked to transfer 284.35 euros. As it's not a high sum many businesses may pay it without questioning it."