This is a good 'un, not seen one like this for ages.
If it's your business, not a bit on the side I can see why you might be peturbed by an undercut, but to complain for a one off seems a bit strong. If you did a few of them, he/she/it/they might have a point.
Stick £1 on top of each set of lines and become a member trader? It'll cover the cost to you and put the whole thing to bed in one go.