If you're a new customer, you can enable the double subscription feature by selecting the 'Save shipping and order 2 months' checkbox on both the Your Pack and Payment pages during the checkout process. Your order total will automatically update to take into account 2 months worth of Vitable packs.

Alternatively, if you're an existing customer, you can enable this feature by logging into your account and navigating to the Shipping & Payment page, where you can select the 'Order 2 monthly packs to save on shipping' checkbox. The double subscription feature will then be applied from your next order.