Form Data not getting stored in database

Hi,

I have created two pages – Contribute Page & Checkout Page.

On Contribute Page I have following code:

<?php
if(isset($_POST['submitnow'])) {
        global $wpdb;
$wpdb->insert(
	'wp9c_treetweet',
	array(
		'name' => $_POST['aname'],
		'amt' => $_POST['aamt'],
		'dept' => $_POST['adept']
	),
	array(
		'%s',
		'%d',
                '%s'
	)

);
}
?>

<form action=""	method="POST" id="addcourse">
<label>Name:<input type="text" name="aname" size="30" required></label>
<label>Amount:<input type="text" name="aamt" size="30" required></label>
<label>Department:<input type="text" name="adept" size="30" required></label>

<input type="submit" name="submitnow" id="addcoursesubmit" value="Submit">
</form>

This code is working fine and is inserting the form data properly in database.

To redirect this page to Checkout Page I have added following code in functions.php:

function redirect_from_checkout() {

   if( !isset( $_POST['submitnow'] ) )

       return;

         wp_redirect( home_url( '/checkout/' ) );

         exit;
}

add_action( 'template_redirect', 'redirect_from_checkout' );

After adding redirect code in functions.php the redirection is working properly but the form data is not inserting into database table.

Plz help me out with a solution.