If your ecommerce deliveries are restricted to certain pin codes (especially in food and grocery categories), then verify the customer’s zip / pin codes as early as possible in the buying journey.
The best time to do it, is when they open the website or app, prior to login. If your site has a pin code check near the “Add to Cart” button, then you can verify it the first time (and only once per session / order) when they add a product to the cart.
Users will be frustrated if they find that their pin codes are not serviced after they have rigorously searched and added many products to their carts. This problem is typically found in ecommerce websites of brick and mortal retailers where the focus on Shopper Experience is still nascent.