App**** promo But was charged full price on my bank account

I made an order using the app on my phone as directed on their website and entered the discount promo of FASHION30 and got the 30% off on non sale items they were running to promote the App. Selected a few items for the kids, got the total and checked out but when I got the bank sms and emailed invoice, I noticed was billed the full amount as if no promo was used. I emailed them and days later they reply and say I am lying. Why would I lie, who sits and thinks of these things? The total before I pressed pay (with the discount) was R923 but R1163 was deducted from my account. I have shopped with them before and shopped online with many other stores. Never been treated like a lair for a mistake by their system. Bad business tendencies. Never shopping with them again, I hear Superbalist is more reliable Be careful of these online shops, not accountable.