MRP Home Online Promo

I made 2 online purchases on 3rd December at the MRP Home store. They have a promo where you buy for 1500 or more you put in the promo code Jingle 300 and it discounts you R300 off your purchase. Due to a technical error the system discounted me too much. However i received emails to say my order is on its way and when I tracked my order it showed my order is canceled, without consulting me first. Just an apology email was sent with a voucher for R600 off each of my purchases, provided that it is over 1500 and must be used by 14 December. Now everything I wanted at the online store is out of stock and to top ot off the vouchers they sent me is not valid. I called the Call Centre so many times and still no joy. There is a nice lady by the name of Norma assisting me, when she escalates my issues I don't receive feedback and have to call all the time, this is so frustrating and so unfair towards me as a customer. Why could they not have called and informed me and just corrected the discount I was supposed to receive and kept my original order, as I said now everything is out of stock and I can't even use the vouchers in a store, its online only. As a loyal MRP customer, I am extremely disappointed and disgusted by this and now I have to struggle and call around.