First, since you have a choice, I would use Access instead of Excel.
- Create your table of users; populate it.
- Create your Form; add controls for sign-in; add ComboBox for "Letter Type"; add command button to "Submit"
- Is the letter going to be customized for the user (their name, address, etc.?) or just a standard form letter? If customized, you will need to create a "Mail Merge" Word document and pass the variables to it (i.e. use VBA code in the "Submit" button) Lots of resources for this if you Google "Access Word Mail Merge VBA"
- How will the letter be sent? Email? If so, there is VBA for that also. Good Luck.