Invincea (the new owner of Sandboxie) has brought back lifetime licensing:
2. Lifetime Licenses are Back!
We heard you loud and clear that many of you want lifetime licenses. We brought them back, so you can pay once and forever have a Sandboxie license.
Apparently this isn't a time-limited special - it's a standard purchasing option.
$35 for one computer, $50 for 3, or $75 for 5