Final answer:
The letter's purpose is to inform the recipient about an unexpected issue with the expense sheet and to update them on this matter without seeking assistance or apologizing for the situation.
Step-by-step explanation:
The purpose of the letter mentioned is primarily to inform about an unexpected surprise within the expense sheet. It serves to update the recipient about a recent development that was not anticipated. The letter also implies a positive assurance that the issuers are dealing with the situation effectively. While the letter does not explicitly request assistance, apologize, or delve into details, it does communicate a significant update that requires the recipient's awareness. The example demonstrates a proactive stance in dealing with discrepancies by highlighting the importance of vigilance in financial matters through the character Noel, who identifies an error and takes immediate action to prevent incorrect payments.