Flutter – IAP expert

We updated our fitness app and asked the dev to add IAP subscriptions. It’s been 8 weeks and still no good.

We have an admin area (php) where price points are loaded.
Users selecting a Pro area are directed to Upgrade page where the Price points load.
Select price option. Pay. On confirmation upgrade user to pro. Redirect to home page.
Receipts need to be checked on login to see if the user is still paid (cancelled, refunded, chargeback). If no payment then downgrade user.

pls start your proposal with API