I'd love to help you enter the complete amount for the completed invoice.
After you have saved the estimate, you will see the Create Invoice button at the top of the of estimate form. Once done, there will be a pop-up window asking How much do you want to invoice?. From there, you can see the Total of all estimate lines.
Open the estimate.
Click Create Invoice.
Fill in the radio button for Total of all estimate lines.
Click the button for Create invoice.
I've attached some screenshots so you can have a reference.
If you have additional questions about estimates in QuickBooks Online, just let me know as I'm always here to help you out.