Buying
Download DiskDuo, sign in with Apple or Google, and buy inside the app. One payment, and every 1.x update is included.
One Mac at a time
A licence is active on one Mac. Signing in on another offers to move it; the first Mac stops the next time it checks in.
Offline
DiskDuo checks in with the licence server from time to time and keeps working offline for up to two weeks between checks. Past that it asks to check in; your scans and settings are untouched.