We would recommend checking your online customer account in the first instance as this will have a breakdown of your parcel item by item.
The total cost of your parcel may increase/decrease due to extra items being included (a folder or special item) or exclude (due to out of stock)
Also its great to bear in mind that the price per parcel may increase depending on which shipment is due, we also recommend checking out the "Your subscription in detail" section on your collection home page which can be find