PHP error: Notice: Undefined index:

I am working on a shopping cart in PHP and I seem to be getting this error "Notice: Undefined index:" in all sorts of places. The error refers to the similar bit of coding in different places. For example I have a piece of coding that calculates a package price with the months a user decides to subscribe. I have the following variables where the errors refers to:

    $month = $_POST['month'];
    $op = $_POST['op'];

The $month variable is the number the user inputs in a form, and the $op variable is different packages whose value are stored in a vriable that a user selects from radio buttons on the form.

I hope that is clear in some way.

Thank You

EDIT: Sorry forgot to mention that they do go away when the user submits the data. But when they first come to the page it displays this error. How I can get rid of it so it doesnt display it?

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This is the code:

<?php
    $pack_1 = 3;
    $pack_2 = 6;
    $pack_3 = 9;
    $pack_4 = 12;
    $month = $_POST['month'];
    $op = $_POST['op'];
    $action = $_GET['action'];

    if ( $op == "Adopter" ) {
       $answer = $pack_1 * $month;
    }

    if ( $op == "Defender" ) {
      $answer = $pack_2 * $month;
    }

    if ( $op == "Protector" ) {
      $answer = $pack_3 * $month;
    }

    if ( $op == "Guardian" ) {
      $answer = $pack_4 * $month;
    }

    switch($action) {   
        case "adds":
            $_SESSION['cart'][$answer][$op];
            break;
    }
?>