You cannot post the seller's ID. There is no official policy prohibiting it, but there are a lot of people who will report the post if you do, and ebay simply removes to appease. That is about the only time ebay will "help in the forum".
I had a hunch that the seller would not actually send it First Class. Her 'promise' to refund any difference in cost did not sound honest to me. But I like to give people the benefit of any doubt, at least in the beginning.
Since you bid on the item with the Priority fee, you may be held to either pay for the item, or get a strike when the seller files an Unpaid Item case against you.
Does the item actually weigh 2 ounces, or could that be an exaggeration? What about the packed weight? (Assuming she does not wrap it in a lead blanket, pack it in cement and mail it in a brick box). It does not likely weigh 4 pounds, however.
Why have you not asked for a cancellation? Have you tried that? If it were me, I would sweetly ask for a Mutual Cancellation and send her the 'how to' link, explaining that it is best for both of us if we just cancel and move on:
http://pages.ebay.com/help/sell/cancel-transaction-process.html
Then, given just how profoundly I despise lies, when the request came in my ebay messages to cancel, I would respond with a refusal. Ebay then closes the case, the transaction is over, I do not have to pay, and she does not get her fees back. Few things are as loathsome as a deceptive cheat.
She may, however, be anticipating such a manoeuvre, and will insist on payment or else she will file the Unpaid case. You may have to pay or get a strike. If you do not get a second one in the next year, you could be okay with it. I have heard of people who will not acquiesce to these kinds of post-sale demands and choose a strike, - careful to not get a second one for the next 12 months.
She is obviously not going to ship it to you for First Class, and seems indifferent to getting a much-deserved negative. I'd say see if she will go for a cancellation and that will get you out from under it.
And keep it sweet, - no matter how difficult that may be at times. Snarly indignation will never work, so be as pleasant as you possibly can.
