Update information about invoicing plugins

TODO

@@ -53,3 +53,11 @@ woocommerce-pdf-invoices-packing-slips:
 -) Uses order_meta: _wcpdf_invoice_number => Set it manually to prevent the plugin from using its own counter
 -) Calls filter wpo_wcpdf_invoice_number($invoice_number, $order_number, $order_id, $order_data)  to format the invoice number. 
 -) Triggers before PDF creation: wp_wcpdf_before_pdf, wpo_wcpdf_process_order_ids, wpo_wcpdf_process_template_order($template_type, $order_id)
+
+woocommerce-pdf-invoices:
+-) Uses order_meta: _bewpi_formatted_invoice_number => Set it manually to prevent the plugin from using its own counter
+-) save function always creates the order_meta => hook in later and overwrite it it non-zero => PROBLEM: There are no hooks called!
+
+yith-woocommerce-pdf-invoice:
+-) Number inserted by a call to get_formatted_invoice_number, no hook available
+-) Number is assumed numeric with only prefix/postfix pre/appended (hardcoded, not with triggers)
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