Using a singleton for a collection of discounts to calculate?

Question

I'm wondering if using a Singleton for a storage of discounts is the right way to go. This is because I will be looping over all added discounts in another class.

client code

// Dynamically add new Discounts
$collection = DiscountCollection::getInstance();
$collection->add(new TenPercentDiscount());
$collection->add(new TwentyPercentDiscount());

DiscountCollection

<?php namespace Notflip\Discount;

use Notflip\Discount\Discounts\Discount;

class DiscountCollection {

    private static $instance;
    private static $discounts;

    final static function getInstance()
    {
        if(static::$instance === null)
        {
            static::$instance = new static();
        }

        return static::$instance;
    }

    public function add(Discount $discount)
    {
        static::$discounts[] = $discount;
    }

    public function count()
    {
        return count(static::$discounts);
    }

    public function show()
    {
        return static::$discounts;
    }
}