The fees are based on the type of a particular service provided to you.  The tax return preparation fees (e.g. for compilation of Form 1040NR) are fixed and based on the type and complexity of the required return, i.e. the totality of all schedules and forms that need to be compiled.  The total fee amount is agreed upon before the commencement of an engagement and then confirmed in the Engagement Letter.  There are no hidden fees.  If after signing the Engagement Letter, you decide to expend the scope of services, you will be furnished with an amended Engagement Letter.
The consultation and audit representation fees are based on hourly rates.  The communications regarding the compilation of your tax return are not considered a consultation. Thus such communications are covered by the tax return preparation fee.