I think your fine as long as in the end, you are selling them a handmade tangible product. It's not like they are purchasing a listing to get nothing in return, there is just a wait time like if it was a custom order. I'd be in trouble if that was the case, I can't really pre-make custom invitations, those also have a good 2-3 week turnaround time as I think quite a lot of handmade items on Etsy have at least that as well. I think you are good to go! I would even put in the listing title that it is a pre-sale for this certain soap, that it will be done and shipped at XXX approximate date, then I agree, in all caps in the description state the time it will take for the soap to be done. You many have a couple of people who don't read and purchase anyways, but there's always gotta be one of those :)