I was not aware we owed you these outstanding invoices and thought we were up to date. These must have somehow remain unpaid by oversight. The sums are small.
We will pay the fees no problem. I will say that to write me message saying all work is suspended until we paid up is not the right approach to take with a client. I'm a lawyer also for 30 years, still active, and can say the I would not have taken this approach in your email with a good client for such small invoices.