Hi Faye,
There have been several different threads that cover these topics in general and specifically. I would recommend taking a look in these sections about
Billing Policies and for
General Billing and Invoicing as there would be a plethora of information available there to give you a better idea of how different VAs handle their estimates, billing and invoicing.
Feel free to add to any of those threads to revive them a bit or to get some current discussion going on them if you still have questions!