Where can I get a list of letters mailed?

The Mail Manifest allows you to view and download a list of all labels generated in your account.  This report includes the date USPS logged the transaction (generated the label), name and address on the label, USPS mail class, tracking number, cost of the label, any unique reference info used, and the user which created the label. 

To view your manifest, navigate to Reports > Mail Manifest. 
You can filter your manifest by mail date, pull up a specific record OR simply click the blue Search button to pull up everything you've done.  

You can download this data as a CSV spreadsheet by clicking the "Download Data" link.  Please note that this is a CSV file and will need to be imported for the Certified Mail numbers (PIC Number) to be expressed correctly. 

See article How to import CSV data into an Excel Workbook to learn how to import a CSV file into an Excel Workbook.