Add-to-next allows subscribers to have products added to their next subscription order, saving money by combining shipments. Products that are eligible for add-to-next will be clearly marked.
If any items are unavailable when your order processes, those items will be removed from that subscription order. You can see what products will be shipping with your subscriptions in your Subscription Manager. You can also remove those products at any time up until your payment method has been charged.
Note that add-to-next does not preserve sale or promotional pricing from the Paizo Store, so only use add-to-next if the shipping savings and your standard subscription discount makes sense to do so. Discounts are shown in your Subscription Manager.