Mix Unit - Order not delivered payment already made

I placed an order for a T-shirt on March **, **** from a website based in US called Mix-Unit. I was notified after placing the order that I will be receiving the product within *-** business days. I waited for a little more than that and when there was still no update, I wrote to their customer care online on April **, **** asking them where I can track my order and when I can expect it to be delivered. I got a reply saying I will receive a link where I can track the status when it's dispatched. I received that confirmation on April **, **** with the tracking link for USPS. From that day till today whenever I click on the link it still shows the same status ie. "Departed USPS regional facility". I waited for a few more weeks and I wrote to them again on April **, **** asking for an update. I received a response on May *, **** saying we have already shipped it and you need to check with your post office. I checked with the post office here in Delhi and was informed that they cannot track any package with the USPS tracking number. So I wrote back to Mix Unit asking them for a refund or at least someone from there contacting USPS to get an update about the delivery since I can't contact USPS customer care as it's based in the US. I still haven't received a response from them. I need a refund for the payment I made as my order was never delivered.