Checkers app problem

Good day, I regularly use the 60 service to deliver my goods from Shoprite Atlantis, and with my last order , I received a message that my order increased and I owe a R12. I was willing to take an item of my list but the sale went through and I didn't think much of it at the time. My problem is this, I don't have any money in my account and the app keep on attempting to deduct money from my empty account and my bank cost is accumulating and increasing by the day. I don't think this is fair because like I said I would've left a product if I have the option to do so to ensure that I don't owe Shoprite but it wasn't given to me. And secondly, when your account is over due to a refund or something the money gets put into your wallet on the account,so why can't you do the same for money owed due to transactions you approve because how I'm I suppose to know after the transaction my total would be more. I'm unhappy because now my bank account is already over R50 in a minus for a R 12 I owe you that wasn't my choice to begin with. And what bugs me is the fact that when you owe them and refund to your bank account it takes up to 3 business day for the money to reflect but when you owe them they want their money immediately