Checkers are daylight ******s

Bought items that were on promotion from Checkers 60sixty app. Discount wasn’t app**** as advertised. Raised the issue with the many times on their app and telephonically with no luck. They didn’t even have the details of their own promotion, as they sent me a DM to tell me they no longer have details of their own promotion and I should send them a screenshot if I have it. That was embarrassing on their behalf, how do you not have history of your own promo, and then turn around and ask your customer to give you details of a promo that was ran by you! Anyway, I gave them the screenshot and the still decided to no refund me the full amount that they owe me! How the ****** heck do you choose to partially refund me my own money??