Per-seat yearly plans on Stripe

Hi there!

I am building a project management tool. My plan is to only offer one paid plan, that will bill "per-seat":

• First project is always free
• Each project after the first is always $3/monthly.

Now I want to offer my users to pay yearly and provide a discount if doing so:

• If paying yearly, each project will be $2/monthly ($24/yearly).

I am trying to set this up using Laravel Cashier and Stripe, which works fine.

My only problem is that if users chose to pay yearly, theoretically they can create as many projects as they like during that year, but only pay for one - as long as they delete their "excess" project before the yearly billing date.

Example:

• On January 1st 2021, "User A" subscribes to my project on a yearly plan. At this time, he have 2 projects and the first is free. ($2usd/monthly)
• During the entire year of 2021, "User A" uses my service and adds 10 projects (Stripe subscription is updated to reflect this)
• On December 31st 2021, "User A" deletes the 10 projects, so he only have 2 projects again. (Stripe subscription is updated to reflect this).

= The invoice being created in Stripe on January 1st 2022 will only be for one project, even though the user have used the service all year with +10 projects

How can this be avoided?