We have received your invoice on the [Services] rendered as of [date]. We were surprised to find that the invoice rate does not correspond to the price quoted on the quotation dated [date].
According to our agreement, your charge / rate / commission should be [amount / %] per [hour / order]. [You will also give us a discount of [amount / %] per [hour / order].] However, the invoice we received showed that you are charging [amount / %] per [hour / order] instead, and the total amount due as [invoice amount] – a difference of [difference].