Yes, if the price was a mistake the retailer can almost certainly cancel the order. This is especially so if they realized the mistake in a short time period before you relied to your detriment on receiving the product at the advertised price. You did not suffer any adverse consequences from this mistake---other than not getting the bargain you hoped for. It would be different if they shipped the telescope to you, you began to use it for a while, and then they tried to bill you more for it----they couldn't do this. But they are permitted to cancel the order in a situation like this.