Dear Sirs, (มี 'S' หลัง 'Sir' คือส่งถึงบริษัท ถ้าเป็นบุคคล ไม่มี 's' นะจ๊ะ)
We would like to thank you for your kind support, and hope that you are satisfied with our service.
On this occasion, please allow us to draw your attention to the outstanding account in the amount of XXX baht, which has been past due since ……(วันที่ถึง due จ่าย)
We look forward to your early response on the settlement of the account, for which please accept our appreciation in advance.
Yours faithfully,