An integrated thank you/social media leaflet to print out after the invoice&shipping label for every order

The possibility to have a leaflet print out on the integrated labels for example 1 ) the postage label 2 ) then the invoice 3 ) then a leaflet type thing

We currently have a leaflet which we include in every order but would be great if we can integrate this to print after each invoice and shipping label, like an uploaded file.

This is to include an offer for future purchases, social media links, contact details etc. To save hand putting in a leaflet for it to print automatically for every order.