Here is one way to go about it.
For Each oCtl As Shape In doc.Shapes
If oCtl.Type = Microsoft.Office.Core.MsoShapeType.msoTextBox Then
oCtl.TextFrame.TextRange.Text.Replace("Invoice", "Invoice - Paid: " + paid_date)
End If
Next
This essentially searches all the text boxes in your document and replaces "Invoice"
As an attempt to find the name of the textbox try this...
For Each oCtl As Shape In doc.Shapes
If oCtl.Type = Microsoft.Office.Core.MsoShapeType.msoTextBox Then
MsgBox("Name: " & oCtl.Name & vbNewLine & "ID: " & oCtl.ID & vbNewLine & "Title: " & oCtl.Title & vbNewLine & "Text: " & oCtl.TextFrame.TextRange.Text)
End If
Next
This should give you unique or identifiable items of every textbox.
then you could just go like this -
If oCtl.Name = "1234" then oCtl.Text.replace(whatever)
or ID or Title or whatever you decide to choose.