You agree to the shipping costs when you buy.  Unless you ask the seller questions about shipping first. 

No do not send until the payment has cleared in your account, except if it is paid by Ping or After pay.

I do all my order's first thing in the morning. My Courier picks up between 8 and 9 am. That way when the buyer is reading their tracking email the  items are usually on their way.  I do tell the buyers if they have emailed, when their items will go out.  I also state in my listings, that when they receive their tracking numbers they can no longer add to their orders. 

If you read NZ Posts Terms and Conditions, you are not allowed to send Jewellery (excluding Costume).  

This happens a lot.  Nothing can be done if items already sold.    But, Yes not meant to list in both places at the same time.

Blacklisting does not stop you being able to see their listings. It stops them being able to hid on your listings.

Just use NZ Post website directly.  Its cheaper.

I have purchased a car from Whanganui and I  am in Tirau before.  My son has bought 1 in South Island and himself in the North Island.   With mine I asked if he would hold it till we got down there. (we  hoped on a plane). no issues.  My son asked the seller if he would take it for a inspection at his cost. Also when he purchased  it he paid half then the car was going on a transporter to get here for him to pay the other half.

Andrew Only NZ Post deliver to Post Boxes.   So Yes you are correct there.  But you can get parcels set to you Post Box.

Very rarely NZ Post have lost any of mine.  If they have not been scanned in, they usually turn up.  The odd one I have had lost NZ Post have come through with compensation.  This can take a while. (also not the full amount you have sold the item for).