the models are not ytours technically.
since you cannot use them for anything else without the consent of the company.

they need the models in case they need different renders or different shots,

they wouldnt want to pay you again to do those new shots.

as for the cost for the models, its questionable.

if it was not in the contract, its up to you do decide wether or not you will charge; but needless to say, wether you give them or not, you cannot reuse them without their ok.

so the models are technicallly not yours.