Hello there, shaun5.
It's nice to have you here in the Community. Allow me to provide additional details on how you can can apply the estimate to a purchase order in QuickBooks Online (QBO).
In QBO, once a customer approves your estimate you can easily copy it to a purchase order.
I'd be happy to share this article about about copying an estimate or quote to a purchase order in QuickBooks Online.
You've got all the information needed. Don't hesitate to reach back to me if you have additional questions while working with estimates and purchase orders. It's my pleasure to help.