To install the Flutter Custom Tabs dependency, which allows you to open web pages within a mobile application, run the following command in your project's root directory:

terminal
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$ flutter pub add flutter_custom_tabs

This will add the line dependencies: flutter_custom_tabs: ^2.4.0 to the package's pubspec.yaml file. The flutter pub get command will be executed automatically.

To use Custom Tabs in your code, import the package in the Dart file where you want to display the checkout:

dart
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import 'package:flutter_custom_tabs/flutter_custom_tabs.dart';

To implement Custom Tabs in your Flutter application, follow the example below, which demonstrates how to open Mercado Pago's checkout in an integrated native browser:

dart
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import 'package:flutter/material.dart';
import 'package:flutter_custom_tabs/flutter_custom_tabs.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: Center(
          child: TextButton(
            child: const Text('Show Flutter homepage'),
            onPressed: () => _launchURL(context),
          ),
        ),
      ),
    );
  }

  void _launchURL(BuildContext context) async {
    final theme = Theme.of(context);
    try {
      await launchUrl(
        Uri.parse('https://flutter.dev'),
        customTabsOptions: CustomTabsOptions(
          colorSchemes: CustomTabsColorSchemes.defaults(
            toolbarColor: theme.colorScheme.surface
          ),
          shareState: CustomTabsShareState.on,
          urlBarHidingEnabled: true,
          showTitle: true,
          closeButton: CustomTabsCloseButton(
            icon: CustomTabsCloseButtonIcons.back
          ),
          animations: const CustomTabsAnimations(
            startEnter: 'slide_up',
            startExit: 'android:anim/fade_out',
            endEnter: 'android:anim/fade_in',
            endExit: 'slide_down',
          ),
        ),    
      );
    } catch (e) {
      // An exception is thrown if there is no browser application installed on the Android device.
      debugPrint(e.toString());
    }
  }
}

You can customize the appearance of the displayed screen by specifying options for each platform. For this, use the CustomTabsOption configuration class, which allows you to define parameters such as colors, animations, and behaviors of the native browser tab. For more details, refer to the official documentation.