Discount dispute and awful way of treating a loyal customer

Last night I went to PnP Promenade mall. I bought 4 baby vests thats on a promo 2 for R50. I didnt ch3ck the slip until today( the next day).That is when I saw that the discount was not deducted, I then inboxed pnp on Facebook. Then they demanded a picture of the promo sticker, which I already threw away in the bin, they told me that they could not help me until I have proof that it was a promo. So I had to go and scratch in my dirty bin was is unhygienic to say the least and I found the stickers, I then sent xthem a pic of the stickers and then nothing. I then said they should keep their f@€£!* discount and that I'm done with pnp. They did not even respond. To think that I shop there every month and one mistake on their part and they treat me like a **** and a dishonest person. That I can assure you I'm not. You were wrong and your stock should be correctly added to your tills and your tellers should watch the discounted products. You have made me very angry pnp, I'm MAD !!! I will not be spending my money at pnp anymore.