I wonder if anyone can help me or point me in the right direction. This is a new request for me.

We are sending out about 10k self mailers a month for a client, who, all of a sudden has asked us to make the self mailers more durable so as not to be damaged (as much) by the Royal Mail machines.

Is there a "5th" colour protective varnish or something I could use as this is a big client of ours and we don't want to lose them.

Any help or advise would be greatly appreciated.