The last sentence in a business letter?

The last sentence of a business letter is the 'thank you' for your time and/or action.

The last paragraph of a business letter is called the 'call to action' paragraph. The last paragraph tells the recipient exactly what you want them to do in response to your letter. It includes any deadlines required, it includes follow up contact information (phone number, email address, etc.). At the end of the paragraph, you thank the recipient. Even if no action is required (for example the letter is simply to inform or update the recipient), you thank them for their time.