Do you have proof of payment, and who decided on the payment amount? If you don't have proof of payments, or the state didn't set up the payment amount, you may be liable for the difference between what you can prove and what they say you need to pay.

I would also consider investigating this child support group that you've mentioned. Is this run by the state of california? I would be wary of anyone who calls and threatens a lawsuit out of the blue. Im also curious of this 30% that you mention. I don't recall any state agency that would withhold 30% of your payments of child support.

The group that demands 30% is Support Payment Collection centers in MN. They charge the Custodial Parent 30% of whatever they collect. Since she signed a contract making them her agent to act on her behalf... I just wonder if I have any legal options and if anyone has any relevant experience with this. She lived in MN when she contacted this group.. so even though she's in CA and the court order is from CA... and I'm in VA... the money goes through MN...

Forgot to add...
Original support order (1994) was for 672.53 for both children.
One is now 19 and not in school.
I have been sending $400 in lieu of a new order.
I have only my canceled checks with "child support" written in the memo line as proof.

I would call the state of California and ask them if they have proof enough that you've paid your child support in accordance with the law based on her income and yours.

Of course they can take you to court, as can anyone with enough time and money, but if you're paid up, they really can't collect anything. I would retain the services of an attorney (a father's advocate group may give you representation pro bono) and initiate a counter suit to collect any attorney's fees incurred to fight these erroneous accusations.

You might want to talk to your former spouse as well; just to verify she has retained these folks to collect whatever amount she thinks is due. At the end of the day, it's the state's decision to collect monies that have not been paid, and its companies like these that like to shake down unprepared fathers with scare tactics.