#7072539 Fix extra Professional Email mailbox purchases

Changes proposed in this Pull Request

• This PR updates the Add New Mailboxes page for Professional Email to correctly support annual subscriptions, both in terms of adding them to the cart, and displaying accurate pricing information for proration, introductory offers, and renewals.
• The PR also depends on D72373-code to supply the product slug for the existing Professional Email subscription

Testing instructions

• Run this branch locally or via the Calypso live server
• Ensure that you have one domain with a monthly Professional Email subscription, and one domain with an annual Professional Email subscription (you can add these during testing if needed)
• For each of the above domains, navigate to Upgrades -> Emails and select the domain from the list
• Click on the “Add New Mailboxes” item when it becomes visible
• Verify that the pricing notice at the top of the page contains wording that reflects the subscription period (it should be “month” for monthly subscriptions and “year” for annual subscriptions)
• Fill out the form to create an additional mailbox, and submit the form
• Verify that you are redirected to checkout and the correct billing interval is displayed in checkout

It is also worth noting that there are three cases that need to be checked for both the monthly and annual products:

• The subscription is still within the free trial period, so additional mailbox purchases are free during the trial
• The subscription is out of the free trial (or has been renewed once), and the expiration date is less than a full billing period in the future, so additional mailbox purchases are prorated and cost less than normal (where a “billing period” is either a month or a year)
• The subscription is out of the free trial (or has been renewed more than once), and the expiration date is more than a full billing period in the future, so additional mailbox purchases are prorated and cost more than normal (where a billing period is either a month or a year)

It may be easiest to mimic these by modifying the back end domains API to manually specify a value and text string that are more or less than normal – ping me for instructions if you’d like to handle things this way.

Screenshots

All of the screenshots below are the new translations for annual subscriptions, as these are all new strings.

Purchase within the free trial period

Purchase that is less than the standard price

Purchase that is more than the standard price

Purchase at the standard price outside of the trial period