I've seen similar styled pop-ups before and thought a heart would work really well.
The card is also really minimal (which I love) and take about 20 minutes to make (that includes the card, insert and envelope).
Step 1: Print and cut insert template
Step 2: Cut and score your card
You can score the card with your craft knife (just don't press too hard). Alternately you can use a compass point or even a pin!
Step 3: Start folding
Step 4: Slowly fold card in half
Step 6: Make outer card
I left mine blank (I really dig the minimal look) however if you want to decorate the outside then you can!