What is the best way to communicate between user and merchant apps in Dart/Flutter?

I need a realtime way to communicate between a merchant and customer app built on flutter. The merchant initiates a transaction which needs to be accepted by the customer on the customer app in real time. What are the best ways to do the same.

I have created the API calls for the merchant action and also for the accept action from customer side. I’m looking for a robust way to send the data from merchant app to the customer app and complete the transaction in realtime. Please help!