working out if a product fits within the max dimensions for a "letter" delivery

australia_post.php

// work out if we're doing a letter or parcel delivery
// Letter max is 26cm x 36cm x 2cm / 0.5kg
// everything larger will be parcel.
$max_dimensions = array(2,26,36);
if($calculate_product['weight']<=0.5){
    $product_dimensions = array($calculate_product['length'],$calculate_product['width'],$calculate_product['height']);
    sort($product_dimensions);
    $is_letter = (
        $product_dimensions[0]<=$max_dimensions[0] &&
        $product_dimensions[1]<=$max_dimensions[1] &&
        $product_dimensions[2]<=$max_dimensions[2]
    );
}