Thank you for all your helpful comments and insights. I am happy to report that things turned out very satisfactorily.
I messaged the seller with photos of the bent sheet, simply informing him/her of the condition the item was received in. I purposely did not ask or request anything because I wanted to see how the seller would react to just a simple factual message. Apparently, that strategy worked well because a few hours later I received an e-mail from eBay
& PayPal indicating that I have been issued a partial refund for the order. The partial refund amount was the full price of the item (in this case the sheet), so in the end I am only out the shipping cost and have some visually stunning significantly discounted postage. The seller must have thought the sheet would be difficult to re-sell or wasn't interested in discounted postage and maybe that's why they didn't offer a full refund on return. The seller is not an exclusively stamp seller, so perhaps they don't have an affinity toward stamps.
I was fully prepared to accept returning the sheet for a full refund if the seller offered that or if the seller offered a partial refund that was too low, I would have refused and initiated a SNAD claim and returned for full refund.
One other idea that came to mind was if the seller had other items I was interested in, would be to ask the seller if he/she would accept a significantly discounted best offer on another item they have for sale in lieu of a return or any refund.
The mailing envelope was not rigid. The sheet was packed in a 10" x 13" polyethylene envelope along with a piece of cardboard (this cardboard was about the strength of a box of cereal cardboard) underneath the sheet. Clearly, not rigid enough. The cardboard needed to be more corrugated and there needed to be cardboard on top of the sheet as well. The sheet was shipped First Class Package.
I'm happy with the resolution.