Well, three days ago was Friday and today is Sunday. Some sellers do not work weekends.
Some sellers just ship without notifying the buyer that it has been done.
When sellers use Paypal shipping labels, the recipient is automatically notified. If not, not.
Unless there has been a drastic change in the seller's feedback or Detailed Seller Ratings, don't worry about it. But mark Day 20 on your calendar for possible further action.