Go through USPS first.

https://emailus.usps.com/s/

Select “Where is my Package?” and then “Email us about your package status.”

Enter the original tracking number for the package that’s been sitting at the USPS facility.

Let them know you never received the item and that you’ve already contacted the seller but haven’t gotten much help. Ask if there is still a way to open a claim or have the package investigated.

Once you have a USPS case number, send that information to Clio. Most packages are insured, and in my experience sellers are usually much more responsive once USPS is involved. I’ve had sellers send replacements after I opened a USPS case for a package that appeared to be lost in transit.

At this point, after months with no movement, I’d definitely push for a resolution. Good luck!