We are writing to let you know that the actual amount payable is 30,000.00 baht. We have so far invoiced you only for 10,000.00 baht as a result of miscalculation.
We would therefore be grateful if you would kindly pay us the balance of 20,000.00 baht.
Please kindly accept our sincere apology for the errors on our part and for causing you inconvenience.