Sometimes we have people that need to make additions/changes to orders and they don't always fit (usually it's postage). Typically it's people who place international orders and choose USPS 1st Class International and don't realize that USPS 1st Class is uninsured, does not track point to point, etc... in situations like that we usually move an order to "pending" and hold it until the individual confirms that they accept the liability for that shipment (because we will NOT replace uninsured shipments). Not to our friends around the world - ordering $200 worth of product and then going with the cheap shipping route = unadvised.
So, long story short, NT bucks allows people to pay for the postage upgrade.
Don't push that button unless you want to give us a tip.