I bought a pack of cards & envelopes from Joann's but you could do postcard style and cut the card yourself (reducing cost). We had poster paint so I used that and watered it down a little which makes a better flicking/spraying action. The heart was cut free hand from another piece of card. I ended up using a couple as they got harder to get off the more painty they got.
Using a little spray adhesive stick on the heart and flick away!
We found just bending the card levered up the heart enough to peel it off put it onto the next one.
Now onto writing those names....
Happy Valentine's Day x