Flutter - How to print a receipt to bluetooth printer?

Bluetooth API is a platform-specific API, so you should use Platform Channel in Flutter to deal with those APIs in the separate platform (Android and iOS). In a word, Platform Channel supports you communicate between Dart module and native module (android and ios)

From Dart module, user triggers an action of printing something, Dart module will send a message to the native module to request printing action, native module calls some specific APIs of OS to do it and when it's done, the native module sends back the result to Dart module to show the result to user.

You should try flutter-blue.dart for bluetooth receipt printer