[Vadik]

Provided that AIF is set up, all you need to do to push sales invoice while posting it is customizing SalesFormLetter_Invoice.updateNow() (see also printJournal() method):

boolean updateNow()
{
    ///

    this.postJournal();

    // ->
    if (custInvoiceJour.canXMLBeSent())
    {
        custInvoiceJour.sendElectronically(XMLDocPurpose::Original, AifSendMode::Async);
    }
    // <-

    #if.never//    #speedtest
    if (speedExecute_Sales)
        speedExecute_Sales.timing(SpeedSalesTiming::InvoiceEnd,'');
    #endif

    return true;
}