First, file a dispute with your credit card company. This is a MUST DO, they are very reasonable. Just explain what happened with names, addresses, and dates of contact, briefly and concisely.
Second, it's worth filing a dispute with PayPal.
Third, file a complaint with the USPS Postal Inspector (or other shipper if different).
If you're still feeling energetic, file a complaint with the state attorney general in both your state and the sender's state. You could also file a complaint with the Federal Fair Trade Commission (FTC) and the US Attorney General. If you and the sender are in different states, you could even file with the FBI and ask if they can investigate it as fraud.
This will take you maybe half an hour for all of the above, and only about 15 minutes for the first three. Once you have a brief, concise write-up of what happened, you just search online for each complaint form and fill it out. Remember, you've pre-paid for all of this with your tax dollars and indirect fees. There are lots of federal and state laws and regulations which the seller may have broached, which works in your favor in recovering your payment.
Save the box it was shipped in, and a screen print of the seller's initial offering (I have a copy if you need it, just DM me).
But at a minimum, file a complaint with your credit card provider, PayPal, and USPS Postal Inspector.