How to handle government filing fee?

[invent_prep]

My gig includes filing an application for the buyer with the government.  The filing requires paying a government fee.  How do sellers receive government filing fee from the buyers without having to pay the 20% commission because the government filing fee is not a service fee?  How do I collect the government filing fee from the buyer?  Do I set up a separate payment requirement?

Edited April 23 by invent_prep