Is the quote fixed?
Yes. Once we have seen the document and quoted, that is the price. If we have misjudged the work, that is our problem rather than yours. The exception is if you send a substantially different document afterwards.
This comes up on Pricing and Editing Rates, where it is answered in the context of the work itself.
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Ask us the actual question
Send the draft, the rubric or the score report with it. You get a real answer and a price before you commit to anything.