function buy_store( $atts ) {

$atts = shortcode_atts(
array(
'steam' => '',
),
$atts,
'store'
);

$steam = $atts[ 'steam' ];

$html = '';

if( !empty( $steam ) ) {
ob_start(); ?>

Steam Store

$html = ob_get_clean();
}

return $html;
}

add_shortcode( 'store', 'buy_store' );