Just had a similar experience with a FHR in Florida (Effie, Sandestin). I thought I selected prepay, but ended up with a deposit instead so I first assumed I selected the wrong payment option. Also, the postpaid reservation was not possible to cancel online, so a chat agent helped me to cancel.
WhenI tried to rebook, and studied the conditions more carefully, it was clear that the ”prepaid” booking was converted into a deposit instead…