Add notifications to an e-commerce existing app + troubleshooting

The app is already up and running (iOS, Android, backend website) but I need to add push notifications when certain events occur. e.g. but not limited to:
- Shop just opened (sent to all customers)
- Shop will close in 2h (sent to all users, sellers and customer)
- Seller X just added product Y (sent to all customers)
- You order is ready for dispatchment, (sent to users with outstanding orders 2h before dispatchment)
- You just received a new order for product X, qty Y (sent to the specific seller)
- Custom message from the admin to be sent via backend free text field (sent to customers, sellers, both) (E.g. the shop will stay closed for X day from tomorrow, please...)

a general troubleshooting of the app will be needed too: e.g. add to cart of more than 1 qty not working, outgoing email not working, ...

please state price and delivery time

the app project page: https://www.freelancer.com/projects/mobile-phone/txtSHOP-special-commerce-app/details