diff --git a/shop/lib/checkout-split.test.ts b/shop/lib/checkout-split.test.ts
new file mode 100644
index 0000000..5d1f4e4
--- /dev/null
+++ b/shop/lib/checkout-split.test.ts
@@ -0,0 +1,42 @@
+import { describe, it, expect } from 'vitest';
+import { splitCart, CartItem } from './checkout-split';
+
+describe('Checkout Split Logic', () => {
+  it('should place purchase items only into purchase group', () => {
+    const cart: CartItem[] = [
+      { billingInterval: 'one_time', selections: {} },
+      { billingType: 'purchase', selections: {} }
+    ];
+
+    const { purchaseItems, subscriptionItems } = splitCart(cart);
+
+    expect(purchaseItems.length).toBe(2);
+    expect(subscriptionItems.length).toBe(0);
+  });
+
+  it('should place subscription items only into subscription group', () => {
+    const cart: CartItem[] = [
+      { billingInterval: 'monthly', selections: {} },
+      { billingType: 'subscription', selections: {} }
+    ];
+
+    const { purchaseItems, subscriptionItems } = splitCart(cart);
+
+    expect(purchaseItems.length).toBe(0);
+    expect(subscriptionItems.length).toBe(2);
+  });
+
+  it('should split mixed cart into separate purchase and subscription groups', () => {
+    const cart: CartItem[] = [
+      { billingInterval: 'one_time', selections: {} },
+      { billingInterval: 'monthly', selections: {} },
+      { billingType: 'purchase', selections: {} },
+      { billingType: 'subscription', selections: {} }
+    ];
+
+    const { purchaseItems, subscriptionItems } = splitCart(cart);
+
+    expect(purchaseItems.length).toBe(2);
+    expect(subscriptionItems.length).toBe(2);
+  });
+});
diff --git a/shop/lib/checkout-split.ts b/shop/lib/checkout-split.ts
new file mode 100644
index 0000000..df6b6b9
--- /dev/null
+++ b/shop/lib/checkout-split.ts
@@ -0,0 +1,17 @@
+export interface CartItem {
+  id?: string;
+  billingInterval?: 'one_time' | 'monthly';
+  billingType?: 'purchase' | 'subscription';
+  selections: any;
+  moduleQuantities?: Record<string, number>;
+}
+
+export function splitCart(items: CartItem[]) {
+  const purchaseItems = items.filter(
+    item => item.billingInterval === 'one_time' || item.billingType === 'purchase'
+  );
+  const subscriptionItems = items.filter(
+    item => item.billingInterval === 'monthly' || item.billingType === 'subscription'
+  );
+  return { purchaseItems, subscriptionItems };
+}
diff --git a/shop/lib/wizard-state.test.ts b/shop/lib/wizard-state.test.ts
new file mode 100644
index 0000000..0e2a6fe
--- /dev/null
+++ b/shop/lib/wizard-state.test.ts
@@ -0,0 +1,110 @@
+import { describe, it, expect } from 'vitest';
+import { handleBaseProductChange, handleModuleToggle } from './wizard-state';
+import { Product, ProductModule, Category, WizardSelections } from './types';
+
+describe('Wizard State Management', () => {
+  const mockCategories: Category[] = [
+    {
+      id: 'cat-1',
+      name: 'Kasse',
+      sort_order: 1,
+      is_required: true,
+      allow_multiselect: false,
+      free_items_limit: 0,
+      preselect: false,
+      resets_others: false,
+      created_at: '',
+    },
+    {
+      id: 'cat-2',
+      name: 'Backoffice',
+      sort_order: 2,
+      is_required: false,
+      allow_multiselect: false, // Radio behavior
+      free_items_limit: 0,
+      preselect: false,
+      resets_others: false,
+      created_at: '',
+    }
+  ];
+
+  const mockModules: ProductModule[] = [
+    {
+      id: 'mod-cloud',
+      product_id: 'prod-gastro',
+      name: 'POSCloud',
+      price: 10,
+      requirements: [],
+      exclusions: [],
+      created_at: '',
+    },
+    {
+      id: 'mod-office-1',
+      product_id: 'prod-gastro',
+      name: 'Office Pro',
+      price: 20,
+      requirements: [],
+      exclusions: [],
+      created_at: '',
+    },
+    {
+      id: 'mod-office-2',
+      product_id: 'prod-gastro',
+      name: 'Office Light',
+      price: 5,
+      requirements: [],
+      exclusions: [],
+      created_at: '',
+    }
+  ];
+
+  it('should automatically remove excluded modules when switching base product', () => {
+    const currentSelections: WizardSelections = {
+      'cat-2': {
+        productId: null,
+        moduleIds: ['mod-cloud'] // User has POSCloud selected
+      }
+    };
+
+    // Exclusion rule: 'prod-small' excludes 'mod-cloud'
+    const mockExclusions = [
+      {
+        product_id: 'prod-small',
+        excluded_module_id: 'mod-cloud'
+      }
+    ];
+
+    const result = handleBaseProductChange(
+      'prod-small',
+      currentSelections,
+      mockModules,
+      [],
+      mockExclusions
+    );
+
+    // POSCloud should be removed
+    expect(result['cat-2'].moduleIds).not.toContain('mod-cloud');
+  });
+
+  it('should respect allow_multiselect = false and only allow one selection in that category', () => {
+    let selections: WizardSelections = {
+      'cat-2': {
+        productId: null,
+        moduleIds: ['mod-office-1']
+      }
+    };
+
+    // Toggle on 'mod-office-2' in a single-select category
+    selections = handleModuleToggle(
+      'mod-office-2',
+      'cat-2',
+      selections,
+      mockCategories,
+      mockModules
+    );
+
+    // Should only contain 'mod-office-2'
+    expect(selections['cat-2'].moduleIds).toContain('mod-office-2');
+    expect(selections['cat-2'].moduleIds).not.toContain('mod-office-1');
+  });
+});