Going for a free billing script is as bad as putting your customer websites on a PII box. C'mon, invest a little and you'll see a better return on your investment. Taking shortcuts right from the beginning is probably the worst thing you can do for your customers.

Now if you don't want to pay for the license upfront, you may want to consider their leased version:

V5 Recurring License 1000 1000 Unlimited $24.95/month
V5 Recurring License 2500 2500 Unlimited $49.95/month

Now I might be way off base here and make the wrong assumption, for that I apologize ahead of time if I misunderstood your post.