I also recommend you contact the seller and let them know you rejected the postage due shipment, but I question how the post office turned this into a Priority Mail shipment. Since the seller used stamps, I am assuming the seller did not use a PayPal or eBay shipping label, so it would not include delivery confirmation. If there is delivery confirmation, the envelope has to be at least 3/4 inches thick (and shipped using the first class package rate) or it has to be priority mail (which is a much higher price).

A padded envelope is not automatically priority mail. Neither is a large envelope.

From eBay's perspective, that envelope is not your until the delivery confirmation says you accepted delivery. And without delivery confirmation, the seller is vulnerable to a claim he or she will lose.