Hi there,
The shipping that applies to subscriptions is set in our Subscription Shipping Profile, and then your one-time products use the shipping settings set in the Shopify Shipping Profiles.
Adding the one-time product will always aggregate the shipping rates, which is due to the current restriction on the Shopify side. Since the one-time add-on product is in a different profile than the subscriptions, adding it to the cart will always pull in that second rate.
Has anyone found workaround?