This has happened to me previously with Optus. They charged me the $5 but I contacted them and told them the app advised it was $0 so they credited it
From our Netflix FAQs:
Yes, you can watch Netflix on more than one compatible device, however, this is dependant on your Netflix plan. Best to refer to Netflix on which plan you have and how to change plans if needed.
So we can have Netflix account for free and watch on any number of screens?
To be really clear, what you've subscribed to via Optus for $0 a month (or $5 according to the confirmation you got) is for Netflix not to count towards your monthly data allowance on your mobile plan. This does NOT give you access to Netflix for free, you're still required to subscribe to Netflix and pay for it yourself. They have 3 plans that allow more and more devices to be used simultaneously the more you pay