Hi!
I've been playing around with the items I'll put in with one of my cross stitches, but I almost always include some kind of card.
Originally I was including a blank, store-bought notecard (I get them at Michael's craft store 6 or 8 for $1.50 in those discount bins), and leaving it blank so that the recipient would have a card ready to address to the person the item was going to, if it was going to be a gift.
Inside that card, I would insert a separate half-card (about 2x3", plain cut cardstock) with a rubber-stamped impression made from a stamp I carved myself, usually that had something to do with the item they purchased. (A dinosaur stamped card if someone bought a Godzilla cross stitch, for instance.) I happened to have those stamps from other crafting I've done, so that was simple.
Recently, especially because of Valentine's day, I've been wrapping the items I send out in plain paper and sending a notecard with a stitched heart on it. I leave the card blank so the gift giver can write their own "to" and "from" on it. And then I send the usual separate rubber-stamped notecard that serves as a business card, too.
But I think it's a process, one I'm still in the middle of -- and when you're talking your first sale, I don't think it's a mistake to include something extra nice. I gave my first special order cross stitch customer an extra little framed cross stitch! (She was a friend as well, though. Can't run a business that way normally!) And your first customer might like to know you're extra grateful for their patronage.