They probably did not send the letter to the "swamp". They probably sent the letter to that land and said it is to go to fiona but when the messangers found she was not at the palace they asked where she be and they was told "go to the swamp".
Also the carriage being an onion they probably was like maybe they were out of pumkins o.O because they know Cinderella and seeing it was made of a food item they probably assumed a fairy made it.
Also the carriage being an onion they probably was like maybe they were out of pumkins o.O because they know Cinderella and seeing it was made of a food item they probably assumed a fairy made it.