I539 Online +I539A by mail

we are a French family. with B2 and I 94 expiration date 07/12/2020.
my husband recently submitted his I-539 online for an extension of his current B2 status.
he paid 370 + 85 in biometric fees. he has his receipt and pending with USCIS.

my question is the following

I would like to attached me and my son (14 years old) like co applicant. so I deposited two I 539A for both.

I filed a new Form I 539 for my husband, explaining that he had already filed one online and paid for it.
Page 2 - Part 3

  • 3a
    I ticked: yes previously and pending with USCIS
  • 3b
    provided the USCIS receipt number,
  • 4 his name
  • 5 Date of filing.

my problem is the following: i don’t want to pay $ 370 more.

can i send them like that and pay only 85 + 85 biometric for my son and me?
if it’s not accepted, will i lose my $ 170? or do they ask for a supplement? or a new form I 539 for me and an I 539 A for my son?

We called USCIS several times to understand how to proceed in our situation. but always different answers and always very confused
Please let me know if there are any solutions or options suitable for my situation. Thank you.