Post an invoice using PurchFormLetter_Invoice

We are a AX2009 shop.

I've a Purchase Order and packing slips are already in AX. How do I go about programmatically posting an invoice for one of the packing slips using the PurchFormLetter class?

I think using the purchFormLetter_invoice.update(...) will do the trick, but I'm looking for the right way to do this.

A more general question is where do I find documentation regarding the use of Dynamics AX functional API?

Thanks.