Did an exchange a few months ago but my return package never reached company so I never got a refund. What can I do now?

In the grand scheme of things, it isnt a lot, but it was a ~$70 package. I ordered some items from a pretty popular store, but they didn't fit well so I exchanged them. The company's policy is that i place a second order and return the first one, so that was what I did. Except, for some reason, my return package got lost in mail and NEVER reached them. I contacted them about it twice back then and they kept saying we'll give you the refund once we get the package, but at this point i dont think that will happen. This was in around October of last year.

What are my options now? Should I file a claim with the store, my credit card, or with USPS? How do I make sure I get my money back? I still have the return package's tracking number and it hasn't been updated since then, but I dont know if that even counts as proof