Part of the reason I don't make many cards is because I feel like the inside of the card and the envelope need something to match. Who wants to read what's on the inside when the outside is so pretty, right? So I used a scrap piece of the ruler paper that wasn't quite long enough. I cut it to have a bit on the left of the heart and it was just the right length!
I just checked and there are only 4 main kits left so be sure to grab it if you are interested!