diff --git a/rest/python/server/routes/ucp_implementation.py b/rest/python/server/routes/ucp_implementation.py
index dd61490..c2bdf58 100644
--- a/rest/python/server/routes/ucp_implementation.py
+++ b/rest/python/server/routes/ucp_implementation.py
@@ -176,8 +176,25 @@ async def update_checkout(
 
   platform_config = None
   webhook_url = await extract_webhook_url(common_headers.ucp_agent)
-  if webhook_url:
-    platform_config = PlatformConfig(webhook_url=webhook_url)
+  
+  # Check for platform config in request body (extras)
+  body_platform = req_dict.get("platform")
+  
+  if webhook_url or body_platform:
+    platform_data = {}
+    if body_platform and isinstance(body_platform, dict):
+      platform_data.update(body_platform)
+    
+    if webhook_url:
+      platform_data["webhook_url"] = webhook_url
+      
+    try:
+      platform_config = PlatformConfig(**platform_data)
+    except Exception as e:
+      logger.warning("Failed to construct PlatformConfig: %s", e)
+      # Fallback to just webhook if possible
+      if webhook_url:
+         platform_config = PlatformConfig(webhook_url=webhook_url)
 
   result = await checkout_service.update_checkout(
     checkout_id, unified_req, idempotency_key, platform_config
diff --git a/rest/python/server/services/checkout_service.py b/rest/python/server/services/checkout_service.py
index 3769aef..609d68c 100644
--- a/rest/python/server/services/checkout_service.py
+++ b/rest/python/server/services/checkout_service.py
@@ -1012,6 +1012,26 @@ async def _recalculate_totals(
       # Fetch promotions once for the loop
       promotions = await db.get_active_promotions(self.products_session)
 
+      # Calculate available methods (inventory hints)
+      available_methods_wrapped = (
+        await self.fulfillment_service.calculate_available_methods(
+          self.transactions_session, checkout.line_items
+        )
+      )
+      checkout.fulfillment.root.available_methods = [
+        m.root for m in available_methods_wrapped
+      ]
+
+      # Check for split shipment support (simulated via platform config)
+      supports_multi_group = False
+      if checkout.platform:
+        # PlatformConfig in models.py is extra='allow'
+        # We check if supports_multi_group is set to True
+        # In a real implementation this would be typed.
+        supports_multi_group = getattr(
+          checkout.platform, "supports_multi_group", False
+        )
+
       for method in checkout.fulfillment.root.methods:
         # 1. Identify Destination and Calculate Options
         calculated_options = []
@@ -1082,13 +1102,26 @@ async def _recalculate_totals(
 
         # 2. Generate or Update Groups
         if method.selected_destination_id and not method.groups:
-          # Generate new group
-          group = FulfillmentGroupResponse(
-            id=f"group_{uuid.uuid4()}",
-            line_item_ids=method.line_item_ids,
-            options=calculated_options,
-          )
-          method.groups = [group]
+          # If split shipment is supported, we can simulate multiple groups.
+          # For testing, we split if there are multiple items and
+          # supports_multi_group is True.
+          if supports_multi_group and len(method.line_item_ids) > 1:
+            method.groups = []
+            for li_id in method.line_item_ids:
+              group = FulfillmentGroupResponse(
+                id=f"group_{uuid.uuid4()}",
+                line_item_ids=[li_id],
+                options=calculated_options,
+              )
+              method.groups.append(group)
+          else:
+            # Generate single group
+            group = FulfillmentGroupResponse(
+              id=f"group_{uuid.uuid4()}",
+              line_item_ids=method.line_item_ids,
+              options=calculated_options,
+            )
+            method.groups = [group]
         elif method.groups:
           # Update existing groups with fresh options
           for group in method.groups:
diff --git a/rest/python/server/services/fulfillment_service.py b/rest/python/server/services/fulfillment_service.py
index 7e2fdc5..717eb1d 100644
--- a/rest/python/server/services/fulfillment_service.py
+++ b/rest/python/server/services/fulfillment_service.py
@@ -18,9 +18,17 @@
 and costs based on the provided shipping address.
 """
 
+from typing import Any
+
 import db
 from sqlalchemy.ext.asyncio import AsyncSession
+from ucp_sdk.models.schemas.shopping.fulfillment_resp import (
+  FulfillmentAvailableMethod,
+)
 from ucp_sdk.models.schemas.shopping.fulfillment_resp import FulfillmentOption
+from ucp_sdk.models.schemas.shopping.types import (
+  fulfillment_available_method_resp as avail_mod,
+)
 from ucp_sdk.models.schemas.shopping.types.fulfillment_option_resp import (
   FulfillmentOptionResponse,
 )
@@ -31,6 +39,34 @@
 class FulfillmentService:
   """Service for handling fulfillment logic."""
 
+  async def calculate_available_methods(
+    self,
+    session: AsyncSession,
+    line_items: list[Any],
+  ) -> list[FulfillmentAvailableMethod]:
+    """Calculate available fulfillment methods based on inventory."""
+    # This sample implementation assumes everything is fulfillable via shipping
+    # but we can simulate unavailability for items with low stock.
+    available_li_ids = []
+    for li in line_items:
+      qty_avail = await db.get_inventory(session, li.item.id)
+      if qty_avail and qty_avail >= li.quantity:
+        available_li_ids.append(li.id)
+
+    if not available_li_ids:
+      return []
+
+    return [
+      FulfillmentAvailableMethod(
+        root=avail_mod.FulfillmentAvailableMethodResponse(
+          type="shipping",
+          line_item_ids=available_li_ids,
+          fulfillable_on="now",
+          description="Available for immediate shipping",
+        )
+      )
+    ]
+
   async def calculate_options(
     self,
     session: AsyncSession,