Your statement is perverse. How you bill your customers in no way dictates how you report your income and expenses to G. You may charge your customers a fixed price with no obligation to break out expenses but, when you file your taxes the IRS has specific rules on how you break out your expenses.
You will probably not be audited but, if you are and you have not complied with their requirements, the heavy burden of defending your actions is on you.
Incorporate or form an LLC. Download the tax forms ahead of time (now would be good) then fill out the entire set for your business pro forma (estimate of what you expect). The exercise will help you understand what records need to be kept in order to properly file your taxes.
Many pay to much tax because they fail to properly identify and document countless legitimate but small expenses throughout the year. Read the tax code, keep up with changes, the time spent will probably be the most profitable time you spend working all year.