The "due" date is about as accurate as predictions for going into labour.
Items from North America usually arrive within 15-20 days.
Items from Europe usually arrive within 10-15 days.
Items from China usually arrive within 30 days.
If your item has passed those more realistic, but still estimated, arrival times, first politely notify the seller asking for date of shipping and service used. Also ask for the delivery confirmation number although this is completely useless to you as a buyer. But it does help make sellers react.
If you don't get a helpful reply, and an offer of a replacement is a scam not a helpful reply , then before day 43 from purchase, open an Item Not Received dispute.
Either the seller or Paypal will refund you. (This is where the DC comes in. Giving that number will prove delivery and protects the seller from scamming buyers.)