Items need to be added to the sidecart before we finish processing your subscription order. A few days before we process subscriptions, we send out a confirmation email that says "In the next week or so, we're expecting to ship [the next volume of your subscription]." At that point, we have already added any available items from your sidecart to your subscription shipment. If you still want to add additional items, there may still be time, but the clock is ticking! Just go through the usual checkout process, and you may see an option to ship the new items with the pending order. If you don't see this option, your subscription order is past the point where we are able to add items to it... but you can still choose to ship those items with the following subscription shipment.