I rang 777 and they said there was nothing they could do and I would have to go back into the shop, which I did but the person who sold me the phone was not there, and won't be back until Thursday. Another salesperson took a photocopy of my receipt (which has the early resign fee crossed out and initialled), as well as a copy of the page of my bill showing the $150 charge. He said they'll get back to me.
Hopefully I'll hear back by Thursday, otherwise I'll be back in there first thing Friday. All I'm hoping is that this will be a simple matter to fix and not something I'll have to try and fight them for.