Christchurch across town postage

Hi everyone. Recently I did some listings for the first time since all trades are using Ping. When I was adding the postage costs I added prices for North and South Islands with urban and rural options for both. I tried to add an across town price as I often get across town delivery addresses. The closest option available for selection for Christchurch was Canterbury which of course covers a wide area and is not covered by the across town option. I put a note on my listing that across town postage is available and showed the cost. I’ve now had my first auction sell using Ping and the buyer has paid the South Island urban cost with an across town address. I will refund the buyer but wondered if anyone had a suggestion as to how I can avoid this in future. I use to be able to just write ‘across town’ in but I can’t for the life of me see how to do that now.