Should my invoices be added to my profit and loss amount automatically when marked as paid or do I need to do it manually?

Hello and welcome to the Community, EJ85

On the cash-based accounting method, income will automatically post to the income account on the P+L when a payment is recorded on the invoice. 

You can check which accounting method (cash or accrual) your QuickBooks is set to by going to the Account & settings > Advanced > Accounting > Accounting method.

The income account that the payment will post to is selected on the item (product or service). To view, go to your Products & Services list (Take me there) and select Edit in the Action column > See Income account.

Check that your reporting period on the P+L covers the date that the payment was made (or, if using the accrual-method, the date that the invoice was raised). 

If you need any further support, drop us a message below. Thank you!

Hi EJ85, apologies - I can see you're using QuickBooks Self-Employed.

In this case, invoices are non-posting and are for reference only (i.e. they won't show on any reports). Any income received from invoices should be recorded separately on the Transactions tab and tagged as Business income. 

The transaction will then automatically post to your P+L based on the transaction date. Hope this helps!