can a notarized letter between two parents be accecpted in place of a court order support

In Florida, child support is established according to a formula, and the courts won't deviate from that formula except for good cause in certain situations. Child support is considered to be the child's right and the parents can't simply agree to a different number if it is not supported by law or facts. However, the notarized agreement can possibly be used as a defense to any child support arrearages if the agreed to number is lower than the Florida Child Support Guidelines figure.

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If the support she's seeking is more than what you've agreed to, it sounds like you should use the agreement to oppose her lawsuit.  Most states will enforce an agreement like that, if it's fair.

I'd recommend taking the letter agreement to a local attorney, and letting him or her handle this for you after reading it through.  I'm guessing that the letter is something the two of you prepared without professional help, and there may be problems with it;  depending on how much time has passesd since it was signed, and what has happened since then, it might no longer help you much.