A price book item will sync to Quickbooks automatically whenever Finish, Invoice, or Pay are clicked on an invoice it is attached to.
However, if you do not want to indicate that a job has been completed, you can still manually push the invoice to QuickBooks. To do this, scroll to the bottom of the job details page in Housecall Pro, under the activity feed, and find the 3 vertical dots menu. In this menu you will see the option to Push invoice/payment to QBO.
Despite not being a completed job, this invoice will now push to QBO.
Note: This action will also allow you to push a $0.00 invoice to QuickBooks Online, but you must have at least one line item on the invoice, that can be $0.00.