A PayPal dispute won't get you banned, that's what they're for. A chargeback
might, though, since that's taking the money from them without recourse.
If the seller used eBay's postage label, or even any online service, then any due postage would've been charged to him. I once sent a package and was charged a few bucks some time later because I allegedly didn't use enough postage (I did, and they eventually reversed the charge because USPS made a mistake.)
So you getting charged here is odd. But I would totally file a dispute with PayPal, make sure to show the receipt of any money you were charged, and the eBay receipt where it shows how much you paid for postage there.
Edit: Seems there's a chance he'll still be charged for the postage in the future.
https://community.ebay.com/t5/Shipping/Seller-refu...