Sims Gas' terrible customer service and unprofessionalism

I ordered and paid for 19kg gas cylinder on their digital platform. I received order and payment confirmatio from the company. When it came to delivery time, the company called to request a payment of R300 which is not on the invoice, stating that cylinders and fuel is expensive. This is totally unacceptable treatment of customers and they must stop online sales as they have different prices. which are contraditory. If they don't update price increases, it is not the fault of customers and they must deliver the goods per invoice. There was no apology from the company.