I called Discover and said I wanted to dispute a charge, gave the date of the order and the merchant name, explained it was an order for a computer that was supposed to ship some months after I ordered it but never arrived and that I had tried to contact the merchant to request a refund but had been denied. That was it, I wasn't too specific about the date it was supposed to arrive. They did ask why I waited so long to dispute it, and I explained it was an order for an item that hadn't been released yet, but that the date of it's release had come and gone with no product arriving. They didn't need any additonial info.

This triggered a dispute in PayPal almost immediately with the description "Item never arrived", in the details it said "We're reviewing this chargeback and we'll contact you if we need more information. You don't need to do anything further at this time". Opened it on June 30th, didn't hear anything from Discover or PayPal, case closed in my favor July 16th.