Import data from workbook into Active Sheet, and Allow manipulation of WB around MsgBox

I am attempting to import data out of a weekly excel sheet, into the master workbook. The goal is to have 3 things ha[[en when the user clicks the button:

1 - MsgBox pops up saying "Be sure to select the correct destination worksheet before clicking OK."
1.2 - Allow user to select correct sheet if they have not, then click OK. If this cannot be done, then is it possible to place a cancel option in the box, so that they can select the correct sheet, then try again?
2 - Import the data from the file selected into the active worksheet.
3- Force a sort so that the format of the sheet matches the rest of the master workbook.


Here is the code that I have so far.

Private Sub BTNimportdata_Click()

MsgBox "Be sure to select the correct destination worksheet before clicking OK."

Dim customerBook As Workbook
Dim filter As String
Dim caption As String
Dim customerFilename As String
Dim customerWorkbook As Workbook
Dim targetWorkbook As Workbook
Set targetWorkbook = Application.ActiveWorkbook

filter = "Excel Files (*.xl*;*.xlsx;*.xlsm;*.xlsb;*.xlam;*.xltx;*.xltm;*.xlz;*.xla;*.xlt;*.xlm;*.xlw),*.xl*;*.xlsx;*.xlsm;*.xlsb;*.xlam;*.xltx;*.xltm;*.xlz;*.xla;*.xlt;*.xlm;*.xlw"
caption = "Please Select an input file "
customerFilename = Application.GetOpenFilename(filter, , caption)
Application.ScreenUpdating = False

Set customerWorkbook = Application.Workbooks.Open(customerFilename)
Dim targetSheet As Worksheet
Set targetSheet = targetWorkbook.Worksheets.ActiveSheet
Dim sourceSheet As Worksheet
Set sourceSheet = customerWorkbook.Worksheets(1)
LRs = sourceSheet.Cells(Rows.Count, "A").End(xlUp).Row
LRt = targetSheet.Cells(Rows.Count, "A").End(xlUp).Row + 1
If LRt = 2 Then LRt = 1
sourceSheet.Range("A1:BZ" & LRs).Copy targetSheet.Range("A" & LRt)
customerWorkbook.Close

Dim objSort As Sort
    Dim Rng As Range
    
        With ActiveSheet
            Set Rng = .UsedRange
            Set objSort = .Sort
        End With
        
            With objSort
                .SortFields.Clear
                .SortFields.Add Key:=Columns("I"), SortOn:=xlSortOnValues, _
                                Order:=xlAscending, DataOption:=xlSortTextAsNumbers
                .SortFields.Add Key:=Columns("C"), SortOn:=xlSortOnValues, _
                                Order:=xlAscending, DataOption:=xlSortTestAsNumbers
                .SortFields.Add Key:=Columns("D"), SortOn:=xlSortOnValues, _
                                Order:=xlAscending, DataOption:=xlSortTextAsNumbers
                .Header = xlYes
                .MatchCase = False
                .Orientation = xlTopToBottom
                .SetRange Rng
                .SortMethod = xlPinYin
                .Apply
            End With
Application.ScreenUpdating = True

End Sub

Any help with this would be greatly appreciated.

Thanks,
DaiyannaGrae

I want to add the option to the MsgBox. The intent for this file is that the user can update each week as the reports are availible, by selecting the appropriate week and importing the data that has been downloaded in an excel document. It will be up to the user to select the correct sheet, as this will be used several times each week for different groups. here is the document that I am working with, and an example of the data that will be imported.

mock week 1.xlsx
ZCN50 Weekly Tracker FY13.xlsm

Thank you for helping! That snippet works like a charm!