I reckon Dazzler has given the best advice - offer to pay for the work at the agreed rate per hour of work - not standing around.
I doubt that an outfit offering to up the price if an invoice is required is going to want the charges looked at too closely in court!
I hope I'm not misquoting you with the slabs of text cutout, but I would have thought the issue of paying GST is an issue for the service provider, not the customer. The service provider should be paying GST on cash transactions as well - if they're honest:wink:.
I do wonder about the honesty of tradies (and others) who offer to do work "discount for cash". Me thinks people who are dishonest with the taxman could also be dishonest with me the customer. But maybe I'm too naive:)