BillyGoat is free to use, including the following features...
Detailed offline mapping for every state and territory. Most other apps charge over $100 for this feature alone.
See your location on the map and moving map features
10,000+ Interactive POIs such as Campsites and Caravan Parks
Pins to mark Points of interest
Import GPX files
Premium features which require a subscription unlocks...
Directions to your destination
Monthly map updates
Backup and restore your trip data
The app is very functional even without the premium features. However a subscription unlocks the full experience. A 7 day trial is included so you can experience all the features without any risk.
Manage your Subscription
Subscriptions in BillyGoat are tied to your Apple ID. Please see this article from Apple on how to manage your subscriptions.
Multiple devices (i.e. iPhone & iPad)
One subscription can be shared across multiple devices sharing the same Apple ID. For example, you purchased a subscription on your iPhone and your iPad shares the same Apple ID as your iPhone. To access the subscription on your iPad, when BllyGoat asks you to subscribe, select "Restore Purchases" to retrieve the subscription from your Apple ID.