Here's one way starting from A19. It assumes that you have a Command Button used to update the invoice and that you're using Combo Boxes. You'll need to ammend the code to include all of your boxes or the particular controls that you're using:
Code:
Private Sub CommandButton1_Click()
Dim LastRow As Object
Set LastRow = Sheets("Sheet1").Range("A38").End(xlUp)
With LastRow
.Offset(1, 0) = ComboBox1
.Offset(1, 1) = ComboBox2
' Repeat As Necessary
' Code will not continue past Row 38
End With
End Sub
Hope that helps,
Smitty