Do I need to purchase editing services in advance?

You do not. 

We outsource our editing services to a much larger firm (Admissionado) that maintains sufficient staff to be able handle demand without "scheduling" things.

So no need to worry about "things booking up." 

You can purchase an editing package at any point - either in advance of when you know you'll want to use it, or whenever you have your materials ready. 

Your confirmation email will contain instructions about how to send your materials over and you will be able to do so immediately upon completing your purchase.