Enforcing boundaries with clients

I have a 3-month executive coaching package — $2,500/month.  I have an executive who purchased it and has paid for July, August and September.  It was clear that I was available for one formal call per month, and ad hoc support throughout the month.  I followed up a few times with her at the end of August and early September and she was “busy” and now wants to slip/roll into October.

I’m wondering how firm to be on the boundaries.  I already did a Model to check my own thoughts on not deciding what was or wasn’t “worth it” or valuable to her.  Thoughts?