Delivery error blamed on client

In the 12th of January I ordered a Special (Combo) from the App as advertised at a specific amount. I got an Invoice stating a different (average) line item BUT still was priced at the higher amount as advertised for the Combo. What was delivered is the cheaper option but what was billed was the more expensive option (combo). When trying to rectify this with AMPLE proof, I am told that I need to check my orders before hand, implying that I made a mistake, even if the proof CLEARLY shows an error. Based on this interaction it would seem that the App can change the order, still bill you for more than the value but give you a lesser item? This does NOT inspire much confidence in the service. The likelihood of the order matching the payment is questionable and no accountability is taken even if the proof is as clear as day.