Hello drowranger80,
In order to have an item added to the sidecart ship with a subscription order, it must be added to the sideart before you receive the confirmation email when we generate the order. If you add an item to the sidecart after receiving the confirmation email, it will actually wait to go with the next order we generate without customer service intervention. I hope this makes it clearer for how to add items to your sidecart in time to ship with subscription orders! If you need to make any changes to the item in the sidecart, please let us know. Currently it is set up to join the order we will create on September 26th.