To pay or not to pay

We went to well-known store yesterday to buy some home decor stuff. On getting home we discovered today that they had not billed us for six out of the eight cushion covers we bought. Now, this is not the first time that the store has goofed-up in their billing. But this is probably the first time that they have goofed-up to the customer's benefit. The discussion at home began as to whether we should tell them about the unbilled cushions or not. If we do, I'm sure we would have to go there ourselves to pay the extra amount and I would be surprised if we even get a note of appreciation from the management. On the other hand, if we dont report it, some poor cashier is going to get penalised for it. After a long debate, we decided that this is not an eye-for-an-eye game where we are trying to get back to them for all the injustice they have done to the customer community. So we decided we will call them and let them know. Let's see how they handle this!