Able to Add Addons after Client signs up to a plan

This would be beneficial as currently when a client wants to purchase an add-on they have to add it while they are subscribing to a plan. It would be awesome to have a feature where a client could select a button under services or via the task board to add additional services that are available for their chosen plan. for instance. we offer additional copywriting, social media & SEO services but only for our 2nd plan. At least with this route, it allows a client to add an addon mid-term if they realise they would need it. if that makes sense? :D