Copy/paste data using checkbox-clear when unchckd & multiple selections paste in next cell

Hi,
I've got checkbox in a worksheet called 'Checklist' and I am using the below code to copy the data in the cells next to the checkboxes when selected into another worksheet named 'Estimate'. The pasted cells are to start from B12. I need to be able to check multiple boxes and have them paste in the next available cells from B12, at the moment they just paste over the top of the current data in B12. I also would like the data cleared when the boxes are deselected, the only code I had figured out clears everything in the worksheet from B12, but I have info in various other columns and rows. I've attached my workbook, please help!!!

Module:

Sub Chbx()
Dim shp As Shape, ws As Worksheet, msg As String, c As Integer
Dim cop
c = 1
With Sheets("Estimate")
cop = .Rows(1).Value
.Rows(1).Value = cop
End With
For Each ws In ActiveWorkbook.Worksheets
If Not ws.Name = "Estimate" Then
For Each shp In ws.Shapes
        If shp.Type = msoFormControl Then
            If shp.FormControlType = xlCheckBox Then
                If shp.ControlFormat.Value = 1 Then
                    c = c + 11
ws.Range(shp.ControlFormat.LinkedCell).Offset(, 1).Resize(, 1).Copy Sheets("Estimate").Cells(c, 2)
                End If
            End If
        End If
Next shp
End If
Next ws
End Sub

Checkbox macros (for each checkbox):

Sub CheckBox3_Click()
Call Chbx
End Sub

Hi humdingaling,

thank you so much for looking into this for me, the only thing is I am avoiding using a formula in the Estimate sheet. Sorry I should have mentioned this to begin with!! Im building this worksheet for some tradies to use who have very basic understanding of excel. The issue with putting formulas in is that they always clear the cell in 'Estimate', therefore clearing the formula. Hence im trying to do it through a macro so they cant just clear it away. Would you have any ideas on how I could fix my macro to do what I need without using a formula in 'estimate'?

Thanks for your help!

absolutely amazing!! thanks you so much humdingaling :D