Here is a sample features of a Formal letter.Dear Mr Patal,I am writing to you to kindly inform you once more about the outstanding invoice from September that still remains unsettled. The invoice no. Is 456, dated 3rd September 2010 for the quantity of 435€We request that you settle this amount immediately, by bankers transfer or check, as for every day that the debt remains unsettled we will now be adding a surcharge of 5% to the total amount, after your promise to pay the unsettled amount before the end of October has been brokenIn the mean time, we will have to freeze any new credit until this debt is settled,Yours faithfully,