You'd only have to pay if 1 she sues you 2 she proves in court that you were at fault in damaging her car--which, however, is likely if you bumped the back of her car the law generally presumes that the rear driver in a collission like this is at fault, since you should have maintained a safe following distance and stopped in time and 3 she can prove the amount or extent of the damage. In regards to 3, she can't simply get whatever amount she wants, but can only recover the amount of damages she can prove i.e. to recover $2,000, she'd have to prove $2,000 in damage, or a cost of $2,000 to repair her car. If you believe that she can likely do this e.g. that you did $2,000 of damage, then yes, you will likely have to pay $2,000 if you do to small claims court.