Load()
Overridden to handle loading the invoice itself as well as the customer and lineitem objects. Also supports 0 as a parameter to load a blank invoice with all objects pre-loaded empty so it can be displayed. Also supports -1 to load the last invoice.

Save()
Overridden to save Invoice, Customer and LineItems information.

New()
Overridden to also load a blank lineitem object and empty customer object. Optionally supports a lnCustPK parameter to preload a customer object.

Delete()
Overridden to delete invoice, lineitems and Tlineitems.