Wrong Size Shoe delivered!!!!

I had placed an order for two pairs of sneakers - All size 4 (UK) but upon receiving my parcel (no issues with the courier services), the one pair was a size 5.5!!! Sneaker Universe doesn’t have a direct contact/help line and they don’t respond to emails! Looking at their refund policy, I have to return the parcel within 7 days but can’t do so because they do not provide return details! Now I’m stuck with the wrong size shoe! I’m satisfied with the quality of the shoes but I can’t wear shoes that are a size and a half bigger! This is so frustrating and ridiculous!