Is there a way to have an employee's time added to an invoice automatically?

Hello there, @Syyeda Aafreen.

Welcome to the Community. I can help you see the single time activity of the employee to show under the salary head of expenses in customer wise profit & loss account.

Good job in creating a single time activity and marking it as a billable. Changing the account type of the product and services will automatically change the type of account where your billable expense will post.

Here's how: 

1. Click Sales.
2. Choose the Products and Services tab.
3. Look for the service name.
4. On the Action column, click Edit.
5. Update the account under Income account (see screenshot).
6. Click Save and close.

That should change the posting to your desired account! Keep us posted if you have any other questions about QuickBooks. Wishing you and your business continued success.