>>There is actually a "support and maintenance fee." But I wanted to make sure that the prosective customer understands that if they do not pay the "annual license fee" the program will stop working.
Charge them an up-front fee for a license that stays live as long as they continue to pay maintenance. You can cover that off with a single clause.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1