We currently have no control over Horses as they are not an 'entity' within Bukkit yet. Once Bukkit updates we may be able to control Horses better. For now, it's just one giant ball of bugs!

Regarding your mistake in the shop. If a senior member of staff is online you can ask them politely to resolve this for you, however normal staff members do not have access to any Eco commands, so they would be refunding you with their own money.