OK. Heard back from the guy at Duet.
After the initial response, I asked the following questions:
1 – Someone on the forums of the Bible software I use is using Duet wirelessly (or Bluetooth?). Does this require a subscription?
2 – If I want the $19.99/year, how do I do that and avoid the $3.99/month?
3 – How and on which device do I access these options?
And got the following response:
I'm going to answer your questions in one.
For Duet Pro you will need an iPad Pro for the monthly or yearly subscription, which one of the feature is to use Duet Pro wirelessly. In app you can choose how to pay for the app. You can unlock a whole suite of features including: tailored algorithms for faster drawing response, pressure & tilt control, multi touch gestures zoom & pan, hover, line smoothing, color calibration and Pixel Perfect image refresh.
We are constantly developing the app, so Pro users will get new features regularly and will receive priority support to ensure they are able to rely on the product in their daily workflow.
If you have any other questions, just let us know!
So I asked:
Do I do this from the iPad? Where is the option for the upgrade?
And he answered:
1. To enable Duet Pro you must tap your Apple pencil on your iPad when Duet is loading. This will enable a button on the top left corner of your loading screen that reads "Pro", tap this button to start the subscription process. If the aforementioned isn't working, you can temporarily change the iPad's name to "iPadPro" - without quotations - in order to force this button to appear.
3. Due to the new Apple refund policy, after the 7 day free trial has ended, Apple will refuse to give you a refund. Since all of our transactions are handled by Apple, we are bound to this policies. In case you feel that Duet Pro isn't for you, please cancel the subscription before the 7 day free trial expires to avoid paying for something you won't be using.
If you have any other question(s) ask away! We're here to help.
I hope this helps y'all. I don't think I want to spring for even the $20/year, but some of you may be interested.