Try this. It uses a alias to determine options
Private Sub PostInvoice_Click()
On Error GoTo Err_PostInvoice_Click
If IsNull(InvoiceBalance) Then
[InvoiceTotal] = Me.InvoiceTotal1
[InvoiceBalance] = Me.InvoiceTotal1
[DueDate] = Me.InvoiceDate + 30
Else
MsgBox "You are about...