First things first......
I absolutely adore the fact that you care enough to ask about pricing to have it repaired, Instead of how to get it fixed without shelling out a nickel. Im recently here from a do it yourself forum, and that is the foremost reason for leaving. ...KUDOS....Well done..
Now, before you panic. Go to the gas station, and THOUROUGHLY vacuum the area in front of the 3rd seat, and clean out any "HAppy-Meal" toys, that may be wedged into the seat . (I have kids too....

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In the "LEGS" of the third seat are Latches, which grab the "Floor Anchors in front of the seat. Each of these "Latches" have a sensor mounted to the top, which detects latched and unlatched. ANYTHING that gets in the way of fully latched will cause EXACTLY your symptom......