Would Like a better integration between books and CRM. Creating an estimate of an opportunity is a bit lame as there are no details at all. It would be much better to user either quotes or sales orders, that way you can switch off Invoices in CRM and use the Invoices in books andc/or Invoices. Now, its useless and requires double data entry: create the potential, create the quote - then into invoice/books and then you have to recreate the item listing.
One can work-around, by replacing the Quote function in CRM with the estimate function in Invoice
However, any updates on the amount of the Estimate are not reflected on the opportunity amount, but any amount updates on the Opportunity are updated on the estimate, and, even worse, overwriting the first line of the estimate.
Suggest you go back to the drawing board.