Online shopping nightmare

I recently decided that I would like to do online shopping at the Pick n Pay online site and collect in store as it seemed very convenient and would save me a lot of time. I decided to test the reliability and accuracy by ordering few items just to see how the process went. Initially, it was all smooth sailing, I filled my cart, picked a collection time, put my card details and got a confirmation that payment and the order were done successfully. However, the day before I am due to collect my order I receive an email stating that order had been cancelled due to payment processing problem. I then called PNP online support asking for clarity on the matter, they advised that I should speak to my bank. which I did and they said all was fine on their side they saw an R2 Authorisation fee pulled and was reversed but nothing else. So I decided to try the order again and the same thing happened. Though this time I noticed that when the confirmation screen came up it only should an amount of R2 to be billed and not the full amount of my cart. I then called the PNP online support again and was told the same thing, that I should contact my bank, when I explained that I had and that I suspected that the problem was from PNP side, she said I need to get a letter from my bank stating this and send to them and was completely rude about the entires situation. I don't see why I need to run around getting letters to validate what I'm saying, implying that I am lying. PNP could not give me a proper explanation as to why the payment was not successful. I have done plenty of online shopping before and once the payment says its gone through and confirmed that is the status. I have never placed an order whereby 2 - 3 days after its confirmed they merchant comes back to me saying there is a payment error. I'm completely dissatisfied and disappointed with the experience I had with PNP Online, especially because I expected it to make my life easier but it seems to be the reverse.