Macro to Copy Column, Open Existing Document, Paste & Save?

the main thing is that you need to have the files in the same folder in-order for this to work.

• the inventory file will be an empty workbook and will be filled as needed by you, all the codes and user UI will be stored in it, so it's going to be an ".xlsm" file.

Sub ShowMe()
UserForm1.show
End Sub

Add customer items to ComboBox1, and Add items to the List box from Today.xlsx when UserForm1 initialize

Option Explicit
Private Sub UserForm_Initialize()
'Add customer items to ComboBox1
    Dim m As Long
    m = 1
    With ComboBox1
    .Clear
    Do While Cells(1, m).Value <> ""
    .AddItem (Sheet1.Cells(1, m).Value)
    m = m + 3
    Loop
    End With
'=========================================================
' 1- Open the source workbook as ReadOnly
' 2- extract the needed data
' 3- close the source workbook without making any changes
' 4- populate the extracted data into the listbox.
'=========================================================
Dim ListItemsR As Variant, i As Integer, ListItemsC As Variant
Dim SourceWB As Workbook
Dim j As Long
j = 1
    With ListBox1
        .Clear
        Application.ScreenUpdating = False
        Set SourceWB = Workbooks.Open(ThisWorkbook.Path & "\Today.xlsx", False, True)
        Do While SourceWB.Worksheets(1).Cells(j, 1).Value <> ""
        j = j + 1
        Loop
        ListItemsR = SourceWB.Worksheets(1).Range(Cells(1, 1), Cells(j - 1, 1)).Value
        ListItemsC = SourceWB.Worksheets(1).Range(Cells(1, 4), Cells(j - 1, 4)).Value
        SourceWB.Close False
        Set SourceWB = Nothing
        ListItemsR = Application.WorksheetFunction.Transpose(ListItemsR)
        ListItemsC = Application.WorksheetFunction.Transpose(ListItemsC)
        For i = 1 To UBound(ListItemsR)
            .AddItem ListItemsR(i)
            .List(i - 1, 1) = ListItemsC(i)
        Next i
        Application.ScreenUpdating = True
    End With
End Sub

Inserting selected items in Inventory workbook from the list box

Private Sub Go_BTN_Click()
Range("b:b").Value = ""
Range("c:c").Value = ""
      Dim i, j, m, d As Integer
      i = 0
      j = 1
      m = 1
Do While Cells(1, m + 3).Value <> ""
If Cells(1, m).Value = ComboBox1.Value Then
more:
      Do
        With ListBox1
        If ListBox1.Selected(i) Then
              Cells(j, m + 1).Value = .List(i, 0)
              Cells(j, m + 2).Value = .List(i, 1)
              j = j + 1
          End If
        End With
      i = i + 1
      Loop Until i = ListBox1.ListCount
      ComboBox1.Value = ""
      ListBox1.MultiSelect = fmMultiSelectSingle
      ListBox1.MultiSelect = fmMultiSelectMulti
      Exit Sub
End If
m = m + 3
Loop
GoTo more
End Sub

exporting data

Private Sub Export_BTN_Click()
 '========================================================================
    Dim SRS As Workbook, CSTMR As Workbook, FilePath As String          '=
    FilePath = ThisWorkbook.Path & "\Customer1Template.csv"             '=
    Set SRS = Workbooks("InventoryTemplate.xlsm")                       '= define Workbooks
    Workbooks.Open (FilePath)                                           '= open Customer1Template
    Set CSTMR = Workbooks("Customer1Template.csv")                      '= minimize it
    CSTMR.Windows.Application.WindowState = xlMinimized                 '=
 '========================================================================
    Dim j As Integer                                                    '=
    j = 1                                                               '=
    Do Until CSTMR.Sheets("Customer1Template").Cells(j, 11).Value = ""  '= define search range in
    j = j + 1                                                           '= Customer1Template.csv
    Loop                                                                '=
    j = j - 1                                                           '=
 '========================================================================
 Dim i As Integer, Fitch As Range, SRCHrng As Range, Cell As Range
 i = 1
 Application.ScreenUpdating = False
 Do Until SRS.Sheets("sheet1").Cells(i, 2).Value = ""
    Set Fitch = SRS.Sheets("sheet1").Cells(i, 2)
    Set SRCHrng = CSTMR.Sheets("Customer1Template").Range(Cells(1, 11).Address, Cells(j, 11).Address)
    Set Cell = SRCHrng.Find(Fitch)
 If Not Cell Is Nothing Then
  CSTMR.Sheets("Customer1Template").Cells(Cell.Row, Cell.Column + 3).Value _
    = SRS.Sheets("sheet1").Cells(i, 3).Value
 End If
 i = i + 1
 Loop
 Application.DisplayAlerts = False
 With CSTMR
 .SaveAs ThisWorkbook.Path & "\Customer1Template-" & Format(Date, "mm-dd-yyyy") & ".csv"
 .Close
 End With
Application.DisplayAlerts = True
Application.ScreenUpdating = True
End Sub

Other code are self-explained

Private Sub Clear_BTN_Click()
    ListBox1.MultiSelect = fmMultiSelectSingle
    ListBox1.MultiSelect = fmMultiSelectMulti
End Sub

Private Sub CLRdata_Click()
Range("b:b").Value = ""
End Sub

Private Sub ComboBox1_Change()

    If ComboBox1.Value = "" Then
        Go_BTN.Enabled = False
    Else
        Go_BTN.Enabled = True
    End If
    
End Sub

Private Sub SelectAll_Btn_Click()
Dim r As Integer
For r = 0 To ListBox1.ListCount - 1
        ListBox1.Selected(r) = True
    Next r
End Sub