False Advertising and poor customer service

placed an order via the app to purchase a sale. there was no restriction on how many i could purchase, but once the order was placed the system split my order and gave me a third of it at the special rate and the rest at full price. when contacting the call centre to get this corrected. at first the agent was helpful and said he needs to speak to his manager ( Mr Mila in customer service). after a 15 minute wait he came back and they suggested that i did not place the order via the app and that is why the price was incorrect. when i questioned why a third of the order was at the special price and the rest at the full price they send i must send them an email to prove how i placed the order and they will see if they will correct it. they did not advertise that there is a limit to how many a customer can purchase and then try and try and blame the me for their system mistake.