For the family where you do not know which tooth is next, or what the box should look like. They choose; the fairies do the rest. Sent by email on the day you pick, with your message. It never expires, and whatever is left over stays on the card.
For the grown-ups
It floats in the corner of every page. If a small person wanders up while you are reading about how the tooth fairy actually works, tap it. The whole site turns into the dullest page in the Land, and stays that way even if they refresh.
To come back, press and hold the acorn for three seconds, or tap it once and type the word acorn. Small people rarely have the patience. On a keyboard, Escape hides the site too.