geekiegeek: You do realise that a so called "cashy" is actually illegal, unless you just mean that you want to pay cash for a full job where the sparky pays the correct gst etc :-)It is not for the consumer to enforce the tax laws. Cash is the only legal tender in NZ (See s27 of the Reserve Bank Act), and any shop or tradesman can legally insist on cash [1]. It is up to the shop owner/tradie to account for GST and tax. If a tradie puts $100 in his back pocket, it is not up to you to ensure that he includes it in his next GST return. OTOH the consumer can insist on a GST tax invoice, if the shop/tradie is registered for GST.
[1] Of course any shop which doesn't accept other methods of payment like eftpos is probably shooting themselves in the foot here.