"Other people can assist you with your visa application. Note that under U.S. law (22 C.F.R. 41.103) you must electronically sign and submit your own application unless you qualify for an exemption. Even if someone else helped you complete the application, you (the applicant) must click the “Sign Application” button, or your application may not be accepted."
in addition to above i read somewhere that third parties not related to applicant cannot even fill online form even if submit button is pressed by applicant.
secondly when is the visa fees actually need to pay , after submitting ds160 or at the time of booking biometrics and interview appointment.