I work (outside of plan audits) on a flat fee basis. What I say is the price is the price. If I misjudged the value of a service, the fault lies with me. I still live up to the bargain I made.
As an attorney and fiduciary on many plans requiring audits, I’m amazed by the courage of audit firms that want additional sums of payment after quoting a flat fee. Where I come from, we call that “chutzpah.” If you say that the audit would cost $15,000, don’t ask for another $5,000. The chances you’re going to get clipped and replaced by another firm are substantially high.
Live with the deal you made.