Allow me to provide a few information about marking as billable items in payroll, StrngrDanger.

I know how beneficial for you to mark an item as billable if you have created it within your paycheck. For now, you can only do this when creating an invoice in QuickBooks as mentioned by @katherinejoyceO. You'll want to use Timesheet entries for good Job Tracking, and includes the Service item column and set your employees to be Paid from Time Data. 

For more information, please check this article for more information: Tracking job costs in QuickBooks Desktop. Additionally, I've also included this helpful article for the different reports you can pull up and get a better understanding of your business's financial status: Track your cash flow in QuickBooks Desktop.

If you have any other questions aside from billable items, please let me know in the comment section. Stay safe!