as far as i know there's no (public) API which is officially supported by Oracle.
First choice to load invoices into payables is usage of tables
AP_INVOICES_INTERFACE and AP_INVOICE_LINES_INTERFACE in
combination with payables open interface import program, which does quite
a lot of validation for you.
In addition you'll have a graphical interface for correcting rejected invoices
stuck in the interface tables.
The invoice interface tables are fairly good documented and you'll find a lot
of samples when searching the web.