Inserts are pieces of paper that are rolled up and put into the barrels of pens. A lot of people design their own inserts, but ofcourse you can also just use an insert that someone else has made. A very popular insert for RSVP MX is Kam’s RSVP Insert, that people often want to change into gradient colors. The easiest way of doing this is opening the insert in Photoshop and following this Photoshop Change Insert Color Guide. I hope it helps.
Darkinerita has below compiled all the inserts he thought were worthy enough on this one page for easy access. Note that some inserts “look” bigger, but they will print out normal size, as they are just in a different DPI. All credits go to UCPSB, PDS, and UPSB members who made these!