To pay your fee, log into your case in CEAC and click the ‘PAY NOW’ button under Affidavit of Support Fee or IV Fee on your summary page. Please note you cannot pay these two fees simultaneously; the online system will ask you to pay them one at a time.
This post explains what this change means for you, what forms to use instead, and how to correctly pay your USCIS fees electronically using Form G-1650 (ACH) or Form G-1450 (credit card).