I FIGURED IT OUT.
In case anyone else finds this in search:
in the Item window you enter all possible prices that you might want to list this item at, including potential auctions and BIN.
Then, when you Submit, there is a dropdown (formerly invisible to me :p) that you select whether to list as auction or fixed price.
You can’t change this in the Listing window after submission, and you can’t change it from Reschedule. You have to delete that Listing, go back to the Item, and go to Submit if you made a mistake.