feat: add custom/not_specified shipping modes with cost input for ML publish

- build_item_payload supports shipping_cost for custom mode with costs array
- Add shipping mode selector: me2, custom, not_specified
- Show shipping cost input when custom is selected
- Backend passes shipping_cost through custom_data to payload builder

pos/services/marketplace_external_service.py

@@ -144,6 +144,7 @@ def build_item_payload(
     listing_type_id: str = "gold_special",
     custom_title: str = None,
     extra_attributes: list = None,
+    shipping_cost: float = None,
 ) -> dict:
     """Convert a Nexus inventory row into a MercadoLibre item payload."""
     title = custom_title or f"{inventory_row['name']} {inventory_row['brand'] or ''} {inventory_row['part_number'] or ''}".strip()
@@ -152,9 +153,11 @@ def build_item_payload(
         title = title[:57] + "..."
 
     shipping_payload = {"mode": shipping_mode}
-    if shipping_mode == "me2":
-        # Let ML determine free_shipping/local_pick_up defaults based on account config
-        pass
+    if shipping_mode == "custom" and shipping_cost is not None:
+        shipping_payload["local_pick_up"] = False
+        shipping_payload["free_shipping"] = False
+        shipping_payload["methods"] = []
+        shipping_payload["costs"] = [{"description": "Envio", "cost": str(shipping_cost)}]
     payload = {
         "title": title,
         "category_id": meli_category_id,
@@ -375,11 +378,13 @@ def validate_items(
         extra_attrs = (custom_data.get("attributes") or {}).get(str(inv_id))
         price = (custom_data.get("prices") or {}).get(str(inv_id), inv["price_1"])
         item_stock = (custom_data.get("stocks") or {}).get(str(inv_id), stock)
+        shipping_cost = custom_data.get("shipping_cost")
 
         payload = build_item_payload(
             inv, images, meli_category_id, price, item_stock,
             shipping_mode=shipping_mode, listing_type_id=listing_type_id,
             custom_title=title, extra_attributes=extra_attrs,
+            shipping_cost=shipping_cost,
         )
 
         try:
@@ -480,11 +485,13 @@ def publish_items(
         extra_attrs = (custom_data.get("attributes") or {}).get(str(inv_id))
         price = (custom_data.get("prices") or {}).get(str(inv_id), inv["price_1"])
         item_stock = (custom_data.get("stocks") or {}).get(str(inv_id), stock)
+        shipping_cost = custom_data.get("shipping_cost")
 
         payload = build_item_payload(
             inv, images, meli_category_id, price, item_stock,
             shipping_mode=shipping_mode, listing_type_id=listing_type_id,
             custom_title=title, extra_attributes=extra_attrs,
+            shipping_cost=shipping_cost,
         )
 
         try: