Consider the following step-by-step guide to configuring a Ballerina package that contains an HTTP service.

1. Create a Ballerina package using the following command.

   $ bal new greetings
   

2. Replace the content of the file main.bal with the following.

import ballerina/http;

type Greeting record {|
    string to;
    string content;
|};

configurable int port = 9090;
configurable Greeting greeting = ?;

service http:Service / on new http:Listener(port) {
    resource function post greeting() returns string {
        string message = string `Hello ${greeting.to}! ${greeting.content}`;
        return message;
    }
}

Here, we created a Ballerina package named greetings that contains configurable variables port and greeting with respective types int and Greeting. These variables are used in the HTTP service where the resource method greeting() provides a configured message.
The variable port is initialized with a default value 9090, which indicates that the configuration is optional. This value will be used for the variable initialization in case the configuration value will not be provided by the user at the runtime.
The variable greeting is initialized with the ? syntax, which indicates that providing a configuration value is mandatory. Therefore, the program will finish the execution abruptly with a runtime exception, if the value is not provided at the runtime.

3. To provide the values for port and greeting through a configuration file, create a file named Config.toml with the following content in the current working directory.

   port = 8080
   
   [greeting]
   to = "Tom"
   content = "Good Morning!"

4. Execute the following command to build and execute the program.

   $ bal run
   

   The output will be as follows.

   Compiling source
        sample/greetings:0.1.0
   
   Running executable
   

   This will start the HTTP service on the configured endpoint 8080.

5. To verify the configuration, execute the following command.

   $ curl http://localhost:8080/greeting -X POST
   

   The response will be similar to the following.

   Hello Tom! Good Morning!