Initial payment incl. activation fee, recurring payment excludes activation fee.

I've got an activation fee of $10. on a join form. There are two optional choices, each, if chosen, adds a charge to the normal join fee.
I want the recurring payment to be the join fee plus any options and exclude the activation fee.
How would i write a function for a parameter on a 2nd. total field for the recurring amount? It would say add options and join amount. or use the 'total' field minus the activation fee field.
Or, is there a better way. The subscriptions plugin looks to not work and not be supported. http://206.130.117.200/~wanderin/join-the-wins/
thank you,
eve lurie