To publish an item as not paid to QuickBooks Online, set the 'Publish to' field to 'Bill' within the 'Receipt Details' page.
The 'Publish to' field is the only one that dictates whether an item has been paid or not.
All paid items can be published to the appropriate 'Cash', 'Cheque' or 'Credit card'.
You can find more information about publishing to QuickBooks Online here.