Help: FillDown doesn't work properly when only one line of data

I'm having problems with the code I've been running in my macro. The problem areas run from " 'Insert Org ID" to " 'Moving columns" (I tried highlighting it red but it didn't work). Essentially, the code pulls data from three text files, adds columns, copies all the data from the last two text files to the first text file, adds and moves columns there, and then moves columns A:H of the final set to the template.

The problem is that when the text files have only one line of data, the commands to copy and FillDown to the End of the Selection end up copying down the ENTIRE worksheet, and then when it goes to copy and paste, it brings up an error saying "Copy area is larger than paste area", and stops the macro.

I have tried multiple times to fix this, but nothing I have done works. I really need this to work whether there is one line of data or 10,000 lines of data, or whatever ends up being the case. Ideas?

Here's my code:

Sub DP_Report()
'
' DP_Report Macro
' Convert DP text files to Excel, formats and copies to template.
'
' Keyboard Shortcut: Ctrl+Shift+D
'
Dim LastRow As Long
Dim Day As String
 
Day = InputBox("Date of File (mmddyy):")
LastRow = ActiveSheet.Cells(Rows.Count, "C").End(xlUp).Row
' Opening Text Files
Workbooks.OpenText Filename:= _
"\\Drive\Folder\dp" & Day & "_113.txt" _
, Origin:=437, StartRow:=1, DataType:=xlDelimited, TextQualifier:= _
xlDoubleQuote, ConsecutiveDelimiter:=False, Tab:=True, Semicolon:=False, _
Comma:=False, Space:=False, Other:=False, FieldInfo:=Array(Array(1, 2), _
Array(2, 2), Array(3, 2), Array(4, 2), Array(5, 2), Array(6, 2), Array(7, 2), Array(8, 2), _
Array(9, 2), Array(10, 2), Array(11, 2), Array(12, 2), Array(13, 2), Array(14, 2), Array(15 _
, 2), Array(16, 2), Array(17, 2), Array(18, 2), Array(19, 2), Array(20, 2), Array(21, 2), _
Array(22, 2), Array(23, 2), Array(24, 2), Array(25, 2), Array(26, 2)), _
TrailingMinusNumbers:=True
Workbooks.OpenText Filename:= _
"\\Drive\Folder\dp" & Day & "_2856.txt" _
, Origin:=437, StartRow:=1, DataType:=xlDelimited, TextQualifier:= _
xlDoubleQuote, ConsecutiveDelimiter:=False, Tab:=True, Semicolon:=False, _
Comma:=False, Space:=False, Other:=False, FieldInfo:=Array(Array(1, 2), _
Array(2, 2), Array(3, 2), Array(4, 2), Array(5, 2), Array(6, 2), Array(7, 2), Array(8, 2), _
Array(9, 2), Array(10, 2), Array(11, 2), Array(12, 2), Array(13, 2), Array(14, 2), Array(15 _
, 2), Array(16, 2), Array(17, 2), Array(18, 2), Array(19, 2), Array(20, 2), Array(21, 2), _
Array(22, 2), Array(23, 2), Array(24, 2), Array(25, 2), Array(26, 2)), _
TrailingMinusNumbers:=True
Workbooks.OpenText Filename:= _
"\\Drive\Folder\dp" & Day & "_5298.txt" _
, Origin:=437, StartRow:=1, DataType:=xlDelimited, TextQualifier:= _
xlDoubleQuote, ConsecutiveDelimiter:=False, Tab:=True, Semicolon:=False, _
Comma:=False, Space:=False, Other:=False, FieldInfo:=Array(Array(1, 2), _
Array(2, 2), Array(3, 2), Array(4, 2), Array(5, 2), Array(6, 2), Array(7, 2), Array(8, 2), _
Array(9, 2), Array(10, 2), Array(11, 2), Array(12, 2), Array(13, 2), Array(14, 2), Array(15 _
, 2), Array(16, 2), Array(17, 2), Array(18, 2), Array(19, 2), Array(20, 2), Array(21, 2), _
Array(22, 2), Array(23, 2), Array(24, 2), Array(25, 2), Array(26, 2)), _
TrailingMinusNumbers:=True
' Insert Org ID
Windows("dp" & Day & "_113.txt").Activate
Range("A1").EntireColumn.Insert
Range("A1").FormulaR1C1 = "1000113"
Range("B1").Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Offset(0, -1).FillDown
Windows("dp" & Day & "_2856.txt").Activate
Range("A1").EntireColumn.Insert
Range("A1").FormulaR1C1 = "1002856"
Range("B1").Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Offset(0, -1).FillDown
Windows("dp" & Day & "_5298.txt").Activate
Range("A1").EntireColumn.Insert
Range("A1").FormulaR1C1 = "1005298"
Range("B1").Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Offset(0, -1).FillDown
' Copy to 113 sheet
Windows("dp" & Day & "_2856.txt").Activate
ActiveSheet.UsedRange.Copy
Windows("dp" & Day & "_113.txt").Activate
Range("A1").Select
Selection.End(xlDown).Offset(1, 0).Select
ActiveSheet.Paste
Windows("dp" & Day & "_5298.txt").Activate
ActiveSheet.UsedRange.Copy
Windows("dp" & Day & "_113.txt").Activate
Range("A1").Select
Selection.End(xlDown).Offset(1, 0).Select
ActiveSheet.Paste
' Closing Extra Windows
Windows("dp" & Day & "_2856.txt").Activate
ActiveWindow.Close
Windows("dp" & Day & "_5298.txt").Activate
ActiveWindow.Close
' Insert Report Type
Windows("dp" & Day & "_113.txt").Activate
Range("A1").EntireColumn.Insert
Range("A1").FormulaR1C1 = "DP"
Range("B1").Select
Range(Selection, Selection.End(xlDown)).Select
Selection.Offset(0, -1).FillDown
' Moving Columns
Range("A1").EntireColumn.Insert
Range("H1").EntireColumn.Copy
Range("A1").EntireColumn.PasteSpecial
Range("C1").EntireColumn.Insert
Range("E1").EntireColumn.Copy
        Range("C1").EntireColumn.PasteSpecial
' Move to Template Sheet
    ActiveSheet.UsedRange.Copy
    Windows("Report_Comparison_Template_1.3.xlsm").Activate
    ActiveSheet.Paste
' Close 113 Window
    Windows("dp" & Day & "_113.txt").Activate
    ActiveWindow.Close