QuickBooks Online does not simply mark an invoice as 'paid', there must be a payment on the invoice. You will have two choices on how to handle this: 


1. Add the payment into the ServiceBox invoice and sync that over to QuickBooks Online which will create the payment in QuickBooks Online and mark the invoice as paid.
2. Add the payment into QuickBooks Online, which will mark the invoice as paid there, then sync, which should mark the invoice in ServiceBox as paid.