If I have an invoice for shipping my car but the manager says that it is still just a quote and is subject to change, is this true?

Question Details:

I am trying to ship a car from one state to another. We signed a contract and got an invoice saying that the broker is to receive $200 for their services (which they have already) and that there is a sum of $325 to be paid to the driver. However, they sent another "final" invoice and increased the price due to the driver by $150. She (the manager) insists that the price on my invoice ($325 due to driver) is subject to change because it is still a quote.

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