One app to handle all purchase restrictions, including pre-orders (pay-later options), quantity limits (minimum of x, maximum of y), limited-time sales (available from x day to y day), pre-announcements (available from x), bundle restrictions (e.g., A and B cannot be purchased together), and customer-specific restrictions (login required, customer tags, quantity each/day). The Cart and Checkout Validation API is used. Order rules can be applied to products, collections, tags, and customer tags.