Tracking actual hours on subscription-based service agreements

[Thomas Smith]

Hi,

I'd like to track hours on services that I offer as a subscription.
Subscriptions are typically paid annually and the work done throughout the
year under the subscription agreement is not billable--the purpose of the
tracking is to help in determining net return (to see if the subscription
cost minus hours actually worked under the subscription agreement yields a
positive return). I also want to be able to generate a report for customers
to show them what they're actually receiving as part of their subscription
agreement.

I'm still learning GnuCash and am pretty inexperienced with accounting as
well.

My first thought was to enter an invoice with the work performed, the
appropriate account, and the hours and full rate, then apply a 100%
discount. This didn't work as GnuCash doesn't appear to allow the posting
of a $0 invoice.

Is there another way that I might achieve in GnuCash what I mentioned above?

Thank you,

~ Tom