Lisa,
I get signatures up front approving the proposal & price and payment terms. Once work is completed, I do ask that the client sign off on the work being completed to their satisfaction. I believe that that is very important.
I find that customers are more friendly and easier to work with when they understand the costs and process up front. It sounds like you have been doing most of this, so absolutely design your workorders to also be used as invoices.
If you send me an email, I can send you a copy of one that I designed for a contractor that I was working with a little while back. I could also assist you in converting it to your business if you feel it might be a good fit.
Hope this helps,
Glen
