No, you should not liable for the costs of their third-party document imaging service; it is their choice to use a third-party vendor and incur those costs, and the fact that they voluntarily chose to do this should not allow them to pass those costs on to you. (One caveat: in rare cases, if a court concludes that you in some way acted in particuraly bad faith or broke court rules in some egregious way, the court, as a sanction, may order you to pay some of the other party's costs.)