As a seller that has had stuff broken in the mail, I can tell you what you need to do. First, hang on to that rod and package! Put it somewhere safe and hang onto it until you have the refund check in hand.
Now, you need to visit USPS website and file an insurance claim. You can do it online and it doesnt take but a few minutes, you will need a copy of the sales receipt showing how much you payed for it (that is what they will pay you). Expect a refund check from the post office in about 30 days.
Be aware, you may get a letter in the mail telling you to bring the broken item and all original packaging, plus sales receipt to your local post office for inspection. So make sure you keep it all!
As an alternative, the seller may just send you a refund and ask you to ship the broken rod and original box back to him, then do the claim himself. This will get you your money back right away, but you will be out the money to ship the rod back to him.