Usually when the message about postcode being not valid comes up, they have put the nearest city into their address ( eg Fred Blogs, 123 abc street, smithtown SYDNEY NSW 2456)

You have to remove the Sydney part, and put in the correct town.

Sometime the spelling is incorrect, or they have put down the wrong postcode themselves.... I use a postcode search site to then check if these details are correct.

I always manage to get the label to go through somehow!  .