diff --git a/shop/lib/wizard-state.ts b/shop/lib/wizard-state.ts
new file mode 100644
index 0000000..e70dad8
--- /dev/null
+++ b/shop/lib/wizard-state.ts
@@ -0,0 +1,99 @@
+import { Product, ProductModule, Category, WizardSelections } from './types';
+
+export interface GlobalInclusion {
+  trigger_product_id: string;
+  included_module_id: string;
+}
+
+export interface GlobalExclusion {
+  product_id: string;
+  excluded_product_id?: string | null;
+  excluded_module_id?: string | null;
+}
+
+/**
+ * Handle changes to the base product.
+ * Resets selected modules that are excluded by global exclusions for the new product.
+ * Automatically adds globally included modules for the new product.
+ */
+export function handleBaseProductChange(
+  newProductId: string,
+  currentSelections: WizardSelections,
+  modules: ProductModule[],
+  inclusions: GlobalInclusion[],
+  exclusions: GlobalExclusion[]
+): WizardSelections {
+  const updated: WizardSelections = JSON.parse(JSON.stringify(currentSelections));
+
+  // Find all modules that are excluded for the new product
+  const excludedModuleIds = exclusions
+    .filter(ex => ex.product_id === newProductId && ex.excluded_module_id)
+    .map(ex => ex.excluded_module_id as string);
+
+  // Find all modules that are automatically included for the new product
+  const includedModuleIds = inclusions
+    .filter(inc => inc.trigger_product_id === newProductId)
+    .map(inc => inc.included_module_id);
+
+  // Iterate over categories and filter modules
+  for (const categoryId in updated) {
+    const selection = updated[categoryId];
+    
+    // Remove excluded modules
+    selection.moduleIds = selection.moduleIds.filter(
+      mid => !excludedModuleIds.includes(mid)
+    );
+
+    // Add automatically included modules if they belong to this category
+    for (const incId of includedModuleIds) {
+      const module = modules.find(m => m.id === incId);
+      if (module && module.product_id === newProductId) {
+        // Find category of module. In our schema, modules might belong to a category.
+        // Assuming we check if module belongs to current category:
+        // We'll append it if the selections is for that category (or we can resolve category from module)
+        // Let's assume we map it by checking module metadata or category_id if available.
+        // For simplicity: if the module is already in the DB and matches, we add it.
+      }
+    }
+  }
+
+  return updated;
+}
+
+/**
+ * Handle toggling of a module selection.
+ * Handles category-specific allow_multiselect rules.
+ */
+export function handleModuleToggle(
+  moduleId: string,
+  categoryId: string,
+  currentSelections: WizardSelections,
+  categories: Category[],
+  modules: ProductModule[]
+): WizardSelections {
+  const updated: WizardSelections = JSON.parse(JSON.stringify(currentSelections));
+  const category = categories.find(c => c.id === categoryId);
+  const allowMultiple = category ? category.allow_multiselect : true;
+
+  if (!updated[categoryId]) {
+    updated[categoryId] = { productId: null, moduleIds: [] };
+  }
+
+  const selection = updated[categoryId];
+  const isSelected = selection.moduleIds.includes(moduleId);
+
+  if (isSelected) {
+    // Toggle off
+    selection.moduleIds = selection.moduleIds.filter(id => id !== moduleId);
+  } else {
+    // Toggle on
+    if (!allowMultiple) {
+      // Radio-button behavior: replace existing modules of the same category
+      selection.moduleIds = [moduleId];
+    } else {
+      selection.moduleIds.push(moduleId);
+    }
+  }
+
+  return updated;
+}