Based off the Method:Classic Opportunities screen listed within an Edit Opportunity screen I see 'Estimates' and 'Invoices' listed.
I agree with you Mahlon...I never agreed with MC presentation either. Why would you have Invoices on an Opportunity. Opportunities are where you are atttempting to GET the sale. It should be "closed" if you have an Invoice - cause then Contact is a Customer.
Just wanted to update the thread and inform you guys I have put in a feature request for our product team to take a look at the Opportunities App Ribbon: Sales Order vs Invoices.
I'll keep you both posted in this thread.
Updating our thread here, Sales Order has now been added to the Opportunity App Ribbon.