If you actually merge the letter to a document, it will save what you typed.
You are doing what I do, merging to a screen preview which can be printed.
This still has live fields.

What I've done is add two buttons to my merge toolbar to lock and unlock
fields. These trigger the following two macros:

Sub FieldsLockAllStory()
'   All Story Field Locker
'   Written by Charles Kyle Kenyon 9 December 2004
'   repaired with help from Jezebel 10 December 2004
   Dim oStory As Range
   On Error Resume Next
   For Each oStory In ActiveDocument.StoryRanges
       Do
           oStory.Fields.Locked = True
           Set oStory = oStory.Next
       Loop Until oStory Is Nothing
   Next
End Sub
Sub FieldsUnLockAllStory()
'   All Story Field Unlocker
'   Written by Charles Kyle Kenyon 9 December 2004
'   repaired with help from Jezebel 10 December 2004
   Dim oStory As Range
   On Error Resume Next
   For Each oStory In ActiveDocument.StoryRanges
       Do
           oStory.Fields.Locked = False
           Set oStory = oStory.Next
       Loop Until oStory Is Nothing
   Next
End Sub

I use a modified version of the padlock button from the forms toolbar for
these (changed color on both and open lock on the one to unlock).

Note if you have a Page X of Y in your header/footer, you'll want to
manually unlock that field or it will be wrong.

Otherwise, you can print as a pdf file.