Put this behind a command button; don't forget to change the items in red to the real objects. An envelope is just like any other report. You will have to change the print orientation and a few minor tweaks but its no big deal.
Code:
Dim strWhere As String
If Me.Dirty Then 'Save any edits.
Me.Dirty = False
End If
If Me.NewRecord Then 'Check there is a record to print
MsgBox "Select a record to print"
Else
strWhere = "[Your Primary Key] = " & Me.[Your Primary Key]
DoCmd.OpenReport "Your Report", acViewNormal, , strWhere
End If