Hello,
Welcome to the QuickBooks community. I'll be happy to assist with your question. QuickBooks Online offers a versatile platform where you can manage your business finances efficiently. The system doesn't include a default account called "cost of sales". When you create an invoice from billable expense, the system will apply a credit to the Billable Expense Income account. You can check the details of the transaction in the Transaction Journal by doing so:
Knowing that there is no default account called "cost of sales", there is a chance that the Billable Expense Income account was renamed. I suggest calling our phone support team at 1-855-253-1536 so a specialist can take a closer look. Our agents are available Monday to Friday, 9 a.m. to 8 p.m. and Saturday from 9 a.m. to 6 p.m.
Let me know if you have another question.