Uber Eats complaint: Promo code issue

We ordered from Mega Bite via Uber Eats, with the intention of using a code provided by Uber Eats entitling us to a 50% discount. When placing our order via the Uber Eats app, I entered my order details and ed the order, at which point the order was placed and my account debited to the value of £35.49, with no option to enter the discount code or payment options. I immediately tried to cancel the order via the app, but was unable to do so, I then called customer services and an automated service told me I needed to cancel my order with the merchant. I then called the merchant directly and was told that they could not cancel the order, I would need to contact customer services. At this point, I called customer services again and was told that they could cancel the order, but I would be charged if the merchant had started to prepare the order, which they had. In addition, I have now paid more for my order by using Uber Eats than I would have had I have gone to the merchant directly. I find this service completely unacceptable and fail to understand how it is this inconvenient to place an order for food using a discount code.