mod(regional,italy): adjusted calculation logic for charges

validation for missing code on mode of payment
UOM from settings
Replace str with cstr
diff --git a/erpnext/regional/italy/e-invoice.xml b/erpnext/regional/italy/e-invoice.xml
index 84b7fff..1c416ee 100644
--- a/erpnext/regional/italy/e-invoice.xml
+++ b/erpnext/regional/italy/e-invoice.xml
@@ -153,7 +153,7 @@
       {%- endif %}
     </DatiGenerali>
     <DatiBeniServizi>
-      {%- for item in doc.items %}
+      {%- for item in doc.e_invoice_items %}
       <DettaglioLinee>
         <NumeroLinea>{{ item.idx }}</NumeroLinea>
         <CodiceArticolo>