No, they cannot get out of paying for charges which they agreed to. In this case, agreement can be shown by conduct or behavior: i.e. the customers requested the changes, were sent overage sheets, and did not then, on receipt of the sheets, cancel the work. Therefore, they contractually obligated themselves to pay: they agreed to pay, even if only implicitly (by going ahead with work with knowledge that it would cost extra), so if you performed your obligations--you did the work--they are then in turn obligated to pay. You can, if necessary, sue them for "breach of contract."(If they signed and returned any of the overage sheets, that would, of course, make proving their consent to the costs even easier--in that case, they explicitly and in writing consented.)Moreover, under the doctrine of "unjust enrichment," someone is NOT permitted to knowingly receive the benefit of another's work without paying for it; that provides a second basis to sue.