Bad customer service

Good day,

I bought a dress from pep and I wanted to exchange the item as I lost the slip i couldn't change the item because it's a sale item. Fair enough, I tried calling pep plus more to change my contact details I'm order to get my reciept however the line is always busy or not in service. I tried calling customer but nobody can assist me there as well. The shop assistant has such bad attitudes when I needed assistance to change my item. How else can forth with a item I dont anymore??