Return went wrong

I got this dress in small, it's beautiful ... but it was too tight for me around my hips, this is not a surprise ... when I buy online it happens. So, I decided to return and I DID!!! The company refunded me before getting the product that I greatly appreciated it... The problem is, for some reason, they didn't receive the item and now they want to charge me. I just want to understand it ... because somebody scanned the barcode to mail it to them, how they don't thrust me?! I'm very upset because I may not have any proof anymore, the only one that exists they already had.

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