CORE-647, already part of 3.2. You can test it out in the beta.
This applies to addon services only, not configurable options. Manage service, click Addons (Only exist if there are addons for this package group), then "Add Addon" button.
It takes the client through the process of creating the addon, and to payment. The addon is "pending" until payment is made, and then the cron will activate it.