Who’s at fault here - the seller or carrier?

• Seller cobbled together a box with cardboard pieces held together by two long strips of gray duct tape, which notoriously doesn’t adhere to cardboard. 
• Items weighed 16 pounds. 
• Box took 9 calendar days to arrive.
• Carrier had to put box in a corrugated plastic tub bc it was literally falling apart at the seams and items were spilling out.
• I'm in Texas where we’ve had recent heavy rains (with flooding and deaths in other parts of the state). The box was wet and the contents inside were damaged. Carrier marked as “visibly damaged” in their system. 
• The seller listed the items as “Very Good,” but I can see old damage and heavy wear and tear that was not in the photos. They are in “Fair” condition at best. 
• Despite the damage, I still need/want the items for a project, so I asked for a partial refund. 
• The seller is saying I need to return the items for a full refund because they see this as mishandling by the carrier.  

I don’t feel I should have to go buy a sturdy box for these items, use my tape and stand in line at the post office because the seller was negligent and things went awry in shipping. I still want the items, I just want someone to make this right. How do I handle?