David,
The general agreement was that invoices, being effectively legal documents, should not be changed once sent. That's fine. The problem is that the 'Post' button on the invoice screen is taken to mean 'Sent', and I believe that this is not necessarily true. [..]
I think you're principally asking for a 'store as draft' button.. Not a bad idea, could be handy when you're negotiating price and discounts as you can roll the invoice as soon as you have the agreement formalised.
/// P ///