Bad servoce

Again it is e and Pick n Pay. While I am still struggling to have my Smartshopper sorted, as PnP tries to blame Tyme bank and vice versa, my invoices are not mailed to me. A week ago I bought two t-shirts at the Pierre van Ryneveld branch. They did not have the one color I wanted, so I took two in the hope of exchanging it at another branch. Could not find it, and slip not mailed to me, so problem. I speak to the staff at the Pierre van Ryneveld branch and ask, since the one t*shirt is still with tags and label, can I exchange for tracksuit pants and pay in the difference. They tell me it is not a problem. After the transaction, I ask my invoice, since this is a NEW ITEM. They first refuse and say they cannot as I did not have an invoice. Which is *******, as this is a total new purchase. Who is going to sort my smarts hopper problem and how can they refuse an invoice????