Please note that if you select your Visa type as J-1 Au Pair on Step 1 - Residency, the system will let you skip the 'Income documents' selection page (make sure to select 'Yes' for question 'I have Au Pair income not reported on annual tax form') and you will be able to report only your income as per the invoices/pay slips you’ve been issued or simply based on the income that you received.
Info: Keep in mind that as an Au Pair you would generally not be required to send any supporting income documents with your Federal and State tax return if you did not receive any.