my procress with the client

what i need is a simple system for the client to request work with billing calculation.

the client adds a task to a task list
i price the task (each task)
then the client accepts the task
then i do it.
at the end of the task i mark the task done

then i want to see a summery of the bill of my client for each month.

i have many clients and also i have two large clients. i am a subcontractor for them.
i record how match each client of my large client have to pay.
so i divide the bill.

i need some permissions i want the client could not be able to change the work that it is accepted by him and just comment on it.

time spent is related to a task.
it is bad for me to do work without the client know how match he will pay at the end.
i must tell hit before i begin to work.

PP currently does not have invoicing functionality.
Freshbooks is a popular invoicing service and I seem to remember it's free if you use it to manage less than 3 clients.