This thing was made with Tinkercad. I found that $250.00 minimum from most Wedding card places was a pain, and since I only needed 20 invitations, I decided to seal the flap with 3D intertwined hearts.
They are small, if you double the size (x,y,z increase to "2") and use a brim of 3mm, it makes the hearts you see in the picture (the brim makes it easier to apply the superglue agent to.)

To get the size that is in the display picture, increase the X, Y, Z double (2) and set the brim to 3mm
This gives you a nice flat area to apply your glue material (I used superglue)