My Dress Is Stuck At Mango Since I Lost The Invoice

I bought a dress for my sister from Mango on October 6, 2023, but I had to return it because it was the wrong size. Sorry, but I lost the receipt. Even though Mango has my purchase recorded and there's clear evidence of it on my credit card statement, they wouldn't let me return the dress. They gave me excuses about it being a seasonal item. Lots of companies let you return or exchange items, or even get store credit, even if you don't have a receipt. But Mango always seems to cause issues in these situations. I need your help with this because Mango's return policy and customer service are not very good.