Они просто так скидку наврядли вернут, можно выбрать дргие вещи взамен тех, что нет, и, если статус заказа все еще In Process, написать им:
Dear Sirs,
I am writing to you concerning my order # [номер заказа], customer # [номер клиента]. Unfortunately, some items I ordered are out of stock by now. Although I received refund, I would rather keep my discount on special offer $15 off $100, so, could you please, add the following items to my order and charge me accordingly, taking into account the special offer.
список вещей (с кодами)
Thank you very much for your time and consideration.
Faithfully yours,
Anetta.