*** Moved from Work Visas forum to Working & Traveling During US Immigration, to be among other threads about EAD application ***
If you applied for I-485 based on marriage and paid the I-485 filing fee, then there is no fee for I-765 EAD application or renewal under (c)(9) category. If you sent payment for the I-765 anyway, then USCIS rightly rejected it. Send the I-765 again, this time with a copy of your I-485 receipt notice, and don't send payment.
From the I-765 form instructions -- https://www.uscis.gov/sites/default/files/document/forms/i-765instr.pdf
Renewal EAD. If this is a renewal application and you are applying under one of the following categories, a filing fee is not required for:
[...]
4. (c)(9) or (c)(16) Any current Adjustment of Status or Registry applicant who filed for adjustment of status on or after July 30, 2007, and paid the appropriate Form I-485 filing fee. If you file Form I-765 separately from your Form I-485, you must also submit a copy of your Form I-797C Notice for Form I-485, as evidence of filing Form I-485 on or after July 30, 2007, and payment of the appropriate form filing fee. If you did not pay the appropriate Form I-485 filing fee because your filing fee was waived or you are exempt from paying it, you must pay the Form I-765 filing fee or request that the filing fee be waived.