As a seller, I often put through refunds for overpaid shipping using Ping. It takes a few seconds to sort it out and is no trouble at all. If the seller has already sent your items in two separate parcels, then fair enough. Otherwise they should be providing you with a refund. I would contact them once your items arrive and if sent together, ask again for a refund. I would say something along the lines of ….I would be grateful if you could reconsider your decision not to refund me for the overpaid shipping. I appreciate that the error was initially mine and that you have offered free shippi...

This happens at least once a week for me. If I happen to see the email before I put the item on the courier, it’s no big deal to enter the new address. However if I’ve already sent the item, I make it the buyer’s problem to sort out.  I just send them an email saying sorry, too late but you can go online and arrange for a redirection. So far, no-one has complained via the feedback system.

Also have a look at using Pass the Parcel in this situation. You can either use their courier bags or print your own label. Sometimes they are cheaper than Aramex or NZPost Courier. I am in the North Island and sometimes I find those small South Island towns are more expensive  with Aramex than NZPost Courier.