Accounting letter

I need a letter that follows these directions as listed below.

In this assignment, you will assume the role of a business owner inquiring about the billing practices of a vendor.

You just received a bill for 16 hours of storage services and 8 hours of coding services from a vendor. You were charged $75 for both types of services. You had agreed on a price of $40 per hour for storage services and $75 per hour for coding services.

You do not want to pay this bill until the problem is rectified; in fact, you already put down a $600 payment, which isn't reflected on the invoice at all.

Your assignment is to write a professional business letter to the vendor inquiring about the bill and the vendor's billing practices. After the letter, please write a short summary of internal control practices that should be in place for the vendor to avoid this type of accounting error. Make sure to discuss how this error could be avoided in the future. Also, please comment on the internal control procedures in place at your company to detect this error.