Print document once per document set

I am trying to configure a page to print only for a set of each documents. Here is the scenario:

I have this envelope page that we only want to print at the start of each client. So we send a bunch of records to get printed and it should sort them by the client code and for each client it should print just one envelope page. Whenever it prints the INCORRECT NR page, we need it to print the envelope, but only once per client. We are showing the print department which envelope needs to be enclosed with the letter so we don’t need it for every record being printed. What would be the best approach to doing this in Connect?

You cannot add that page during content creation as at this point the documents are not grouped and sorted by clients.
So you have to output first, then add the envelope page for each document set.