If the prices for those items or services had been posted or otherwise available to you at the time you placed the order--such as on an order form; on a website; in a quote or proposal; etc.--then if they accidently billed you less than they were supposed to, they could correct those prices now. A billing error does not bind them to a lower price, if you'd had notice of the correct price. Or if you would have been eligible for the lower price if you did something which in fact you turned out to have not done (e.g. they were volume discounts--but you didn't order the volume you'd indicated you would) they could correct the prices.But if you were charged what was the correct price at the time, they may not now decide they should charge you more.