I believe that this link provides good information on corporate advance fees: http://www.bankruptcylawnetwork.com/my-mortgage-payment-history-has-a-fee-charged-called-corporate-advance-what-does-that-mean/
Now, to dispute these fees, send a qualified written request to your loan servicer (the loan servicer is that party that services your loan and sends you billing statements). Put your loan number and other contact information on the letter. When you send it make sure that you make a copy and sent it certified mail return receipt requested. Make sure you entitle the letter a qualified written request and specifically describe your dispute, ask what the fees are for, request verification, and you can go so far to request that they be remove from your account. It would be a good idea to reserach QWRs further before sending one. The Bank should acknowledge your request within 20 days and respond to your inquiry within 60 days. If you have problems getting a response, or the bank does not properly respond, then you may contact an attorney. QWRs are governed by the Real Estate Settlement Procedures Act ("RESPA"), and RESPA allows for statutory damages, costs, and attorneys' fees for bringing suit for non-compliance.
Here's a sample letter. http://portal.hud.gov/hudportal/HUD?src=/program_offices/housing/ramh/res/reslettr
The information in this answer is not intended as legal advice nor do I intend to create an attorney-client relationship with any reader simply by answering this question or contributing as a member of AVVO.
Nationstar is a mortgage servicer. It is extremely likely that it does not own your loan, but services it for some undisclosed owner. Mortgage servicers often make substantial extra income adding bogus or improper advances and fees to serviced loans. You should find an attorney in your area who specializes in financial services litigation to look over what is going on and to assist you, ias you are not likely to get anywhere with this by yourself.
Please note that the above is not intended as legal advice, it is for educational purposes only. No attorney-client relationship is created or is intended to be created hereby. You should contact a local attorney to discuss and to obtain legal advice.