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. Please follow the flow shown below and have faith that the update is free!”
STEP 1: Navigate to the Model 15 page of the App store and click "$29.99".
STEP 2: Click "Buy App"
STEP 3: When prompted log in with the same apple ID that Model 15 for iOS was purchased with.
STEP 4: When prompted click "Buy"
STEP 5: When the pop-up confirming that the update is free appears click "OK"
Your free download should now begin.