well, at least an apology:

Dear Dr Ryan,

Thank you for your recent email. Please accept my apologies for the confusion caused by your present invoice. The invoice is printed on the first of each month and sent on approximately the 15th. You have paid over the phone by card for the amount of £65.66 on 01/02/05 and also £43.22 on 11/02/05. The £65.66 has been received (dated 04/02/05) however the £43.22 is still being processed. Therefore your invoice is correct, however only at the time of printing on 01/02/05. Once your payment of £43.22 is received you will only owe the total new charges on the invoice which is £53.90 as you indeed state.

Yours Sincerely

XXXX XXXXXXXXX

For and on Behalf of

XXXXXXX UK LTD