Custom Field IF/ELSE PHP

You mean something like this?

$customhref = get_post_meta( $post->ID, '_ct_text_4dc9e9f74d000', true );

if ( $customhref ) {
?>
     <a href="https://wordpress.stackexchange.com/questions/16913/<?php echo $customhref; ?>">Go To Store!</a>
<?php
} else {
     echo 'N/A';
}