I think that invoices can indeed be edited, as they should be, and let me provide an example for you.
Let's say that you go through the estimate/proposal phase with a potential client. After a few rounds back and fourth they eventually settle on a plan for implementation, click, and then this converts the estimate into an accepted invoice.
But then suddenly, the customer realizes that they really don't wish to purchase one of the itemized items, or want an extra license or two while purchasing software, since the invoice is still in 'pending' status, it should indeed be able to be modified.
Once it is paid, then the job/contract begins, and the invoice is marked as paid. Further, it is indeed no longer able to be modified at this point now that it has reached this statement status. Any further modifications to the contract will, as you had hoped, need to be addressed via either the addition of another estimate/invoice, or if the removal of an item is requested, then a credit memo must be issued to reflect this (since the first invoice is no longer active - it has been paid). The next statement will show the added items as well as the credit, which is how it would be reflected in your accounting/ERP software, if you don't already have it integrated with HostBill.
I hope that helps.