Angelika Zen, I see three possibilities. 1) The seller hasn’t dropped it off at the carrier, 2) The seller set it out for pickup and someone stole it before pickup, or 3) the carrier lost/misplaced it before scanning it in.
Unfortunately, I don’t think you can do anything but message the seller until it hasn’t been delivered by the stated date. Since it has been three days from your previous message, I’d send another one. If it was a small package, it might be helpful to contact the carrier. It would be easier for a small package to fall behind something and not be noticed.
My keyboard is broken. I keep pressing "Escape", but I'm still here.