This update is free if you have a previously purchased version of Model D for iOS connected to your Apple ID. Unfortunately, this is not clearly shown on the Mac OS app store.
If you previously purchased the iOS version of Model D you can install the macOS version for Big Sur through the following steps.
STEP 1: Ensure you are logged in to the app store with the same apple ID you used to purchase the iOS version of Model D.
STEP 2: In the lower left-hand corner of the window click your account name to go to your account page.
STEP 3: Click "Mac Apps" to show your previously purchased software.
STEP 4: Navigate to the Model D app and click the cloud icon to download and install.
NOTE: This software requires macOS Big Sur to run.