Bad customer service

Buying various 25litres of chemicals from this store more than 2 years for all 3 of my businesses. Unfortunately I had a bad experience today and will not visit them again. The cashier made an error by selecting the more concentrated dishwasher on the pos system. I paid the higher price, but got the cheaper product. Being in the food industry, I can only use the less concentrated product, so I was happy I received the correct product. I was already on my way to deliver this (10 minutes after the purchase) when I noticed that I was overcharged and phoned the store. I took pics of both the slip and the 25l drum, but they insisted that I turn around for the refund. I do not want a refund, just an acknowledgment of the credit for future purchases. They are not willing to negotiate on keeping a credit record at the store. I was running out of time and did not turn back for something that could have been sorted telephonically and they insisted I have to bring in the slip and product. My staff need the dishwasher and I do not have the time to drove back for this. I am very disappointed and will now have to search for a similar outlet for future purchases after receiving such bad costumer service.