Thank you. I will wait a while until the crack really affects the performance, I still have 2/3 warranty period. From what I read the replacement does not extend the original warranty period.
PayPal free shipping would apply to the NEW PART that will be coming under the warranty claim, as the new part's purchase price will be refunded by rockauto after getting the old part. (rockauto shouldn't charge outbound shipping on warranty claims)
Then when you request free return shipping from PayPal to send the defective part back, you will be pulling up the NEW PART's invoice, and then select "free return" on that. NOT on the original order! It might be a bit confusing, but what you do on Rockauto and PayPal are different.
So it's more about buying the replacement with PayPal and getting free return shipping on the new invoice (and rockauto refunds the new invoice after getting the bad part, and PayPal will see that refund on their system to confirm the return), and not the original invoice.
I think I said it correctly but it sounded clear as mud, so do read the fine print.