Plans (Storage): Wrong values and broken CTA on upgrade plan in admin when storage previously purchased

[chriskmnds]

Quick summary

When purchasing one of the storage add-ons for a Creator site and going to /plans/[site], the values shown in the storage dropdown for the upgrade plan (e.g. Entrepreneur) will show wrong/ambiguous storage volumes and lead to malformed CTA (with items in cart not permissible for purchase).

With a site on Creator + 50GB additional storage purchased: (100GB selection should really be 150GB, and 150GB should be 200GB):

This is due to a couple of reasons and also points to some intricacies in the implementation:

1. There is a presumably hard restriction that only a single storage purchase is permissible (so storage selection shouldn't really be shown in this case). This needs some clarity, as the error shown during checkout is that additional storage should be higher than what was previously purchased.
2. Irrespective of restriction, the storage upgrade volume rendered for the selections should be an addition/accumulation over the site's storage, not a static value. This can easily manifest itself when we allow selections for comparison, or loosen the restriction, or extend the storage limits on a site (so purchasing an additional 100GB over previous 50GB is allowed), or in regressions like the above spilling into the UI

Steps to reproduce

1. Create a site on the Creator plan
2. Go to /add-ons/[site] and purchase a 50GB storage upgrade
3. Go to /plans/[site] and select any of the options in the dropdown for the Entrepreneur plan

What you expected to happen

1. No storage selections be shown for upgrading to Entrepreneur
2. Whatever renders to reflect an addition to existing storage of a site, not static/default values

What actually happened

Wrong values rendered altogether: "upgrade to 100GB" when the site's already on 100GB

Impact

Some (< 50%)

Available workarounds?

No but the platform is still usable

Platform (Simple and/or Atomic)

No response

Logs or notes

No response