If he had originally said "If you send me the correct items and a return shipping label, I'll return the wrong items" and they sent him the correct items without a shipping label, he's not a jerk for keeping them. But changing the deal after it's made makes him a jerk.
Is the sky brown on the planet of the assholes? You would never, ever pay return shipping on something you didn't order in exchange for something you did. Never. That is always the retailer's fault. They always have to provide return shipping. Unless you explicitly state otherwise, that is the default mode of operation. Further, this is ordinarily handled by including the return shipping slip in the top of box with the correct merchandise, so that the buyer can simply open the box, slap a self-adhesive label on the old shipping box, and send it away with the delivery agent.
What we're talking about here is an etailer acting incompetently and sending product which will not fulfill the original order's purpose, then continuing to act incompetently by not providing return shipping instructions or labels to the extent that they made it clear that they were not interested in receiving the original goods. Eventually they will have likely written this off as some form of theft not worth attempting to recover, which will have been fraudulent as it was entirely due to their failure to do business in the standard fashion.
That doesn't make anyone in this scenario anything but incompetent, except for you. Because you're calling someone a jerk for expecting business to be done as usual, you're an asshole.