We are awaiting the reopening of First Class and Priority Mail post for Australia and New Zealand. We still do not have the ability to purchase post office postage and ship these parcels and we are checking daily. In the meantime, these parcels will be waiting for that change.
In many cases, I can use UPS for these shipments. If I can, you will be contacted by me by email for approval if there is an additional cost (there will definitely be an additional cost for parcels under 2 pounds weight).
As all the carriers have had restrictions placed, it is difficult to know the exact postage when calculating the estimate in the store. There are widely variable rates and service levels available.
I will recommend that you add to your shipment if I can ship it by UPS or DHL. The U.S. Post is only accepting Express Mail parcels (think UPS 2-3 day/FedEx 2-3 day pricing, but it takes much longer).