The simplest way to discover whther you are paying for the stacker or not is to comparethe Unit Entitlements of your unit with another that’s roughly the same size or value and has a car slot. If they’re the same, then you are being over charged and the way to remedy that is to have the Unit Entitlements adjusted (which is, as Kaindub suggested, notoriously hard to do).
However, the maintenance of the stacker will be a line item in your scheme’s annual budget so that will give you a starting point to “reverse engineer ” the Unit Entitlements (UEs) to discover if the savings you and other owners in a similar situation might make would be worth the expense and hassle of taking this to NCAT for adjudication.