This update is free if you have a previously purchased version of Model 15 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 15 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 15.
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 15 app and click the cloud icon to download and install.
NOTE: This software requires macOS Big Sur to run.