I just listed these funny, vintage sticker sheets in my Etsy shop. They are from a teen/kid magazine from the 1970s and '80s called "Dynamite."
From what I can find online, they came with or through the book order forms we got at school. I didn't recognize the stickers right off the bat, but once I saw pictures of the magazine online, I remembered them. I'm sure I had the covers of some of those magazines taped to my wall!
I also listed a set of tiny hearts made from handmade paper. And, the "With Love" stickers are still listed. Maybe they'll be just what someone is looking for to make their Valentine complete!