Currently, you can only create NEW estimates and invoices on opportunities.

If you create them from the contact view, they won't show up on the opportunity. Let's say you accidentally create an estimate from the contact view or the dedicated estimate section—you have no way of associating that estimate with an opportunity. While the contact view shows all associated estimates and invoices, there's no option to attach them to the opportunity after they're created. This forces you to always remember to create them directly on the opportunity.

Fix: Allow linking of invoices and estimates created from any view to the corresponding opportunity, or automatically show the estimates and invoices on the opportunity regardless of where they were created.

(P.S this also means it is NOT possible to create estimates from the mobile app. Currently, you can't even create an estimate from an opportunity. soooo)