Insert terms here. You reached an agreement on something at that first meeting -- after all the artist accepted your doll and you took her merchandise with a promise to promote it. We're going to presume a contract was formed (although that might be incorrect if you never agreed upon financial compensation). In any case, contract or not, we think the result would be the same if the case went before Judge Judy. She'd probably seek to put the parties back where they were before the arrangement was made -- a remedy known as restitution. In other words, you'd return the dolls to each other and each pay the respective shipping costs.
What about the demanding artist? You have no legal obligation to abide by the artist's demands unless you're seeking a speedy resolution and the return of your doll. If you do decide to exchange dolls, we'd suggest you exchange paperwork confirming the terms of the resolution. And of course, if you want to rep an artist in the future, we'd suggest using a rep agreement like the one we've included in our Craft Artist's Legal Guide.