When you enter the main form record, and move into the subform to enter the
line items, the header record saves into its table. Consequently you can't
just "undo" the record and whatever line items you have entered in the
subform.

Perhaps you could add a Delete button to the main form, and use cascading
deletes to delete the line items in the subform as well:

1. Open the Relationships window (Tools menu, or Database tab of the ribbon
in A2007). Create a relationship between tblInvoice.InvoiceID (the primary
key) and tblInvoiceDetail.InvoiceID (the foreigh key), and check the boxes
for:
    [ ] Referential Integrity
    [ ] Cascading Deletes

2. Open your main form in design view, and add a command button with these
properties:
   Name        cmdDelete
   Caption      Delete this invoice
   On Click    [Event procedure]

3. Click the Build button (...) beside the On Click property.
Access opens the code window.
Set up the code so it looks like this:

   Private Sub cmdDelete_Click()
       If Me.Dirty Then
           Me.Undo
       End If
       If Not Me.NewRecord Then
           RunCommand acCmdDeleteRecord
       End If
   End Sub

Clicking the button will now delete the invoice, and the cascading delete
will delete any line items in that invoice.