These are supplementary postage contributions. If your order is larger or heavier than the standard parcel, there may be a requirement to add a supplementary payment to cover the cost of shipping.
You are very welcome to check in advance of an order whether that will be necessary. Simply contact us with the list of parts, and your location, and we'll be happy to help.
In the event that an order already placed requires additional shipping, then you'll always be given the option to take a full refund instead, if you prefer. In that instance, refunds are issued straight away without quibble.