Ebay warning

[roce4e52]

Last week I made an offer on a card for $18.50, shipping was free and stated that both on the offer page and the seller's item page. The buyer accepted my offer but added $2 for shipping. Ebay automatically accepted the offer for me and invoiced me for the price of $18.50 + $2 or $20.50. They did not give me a choice to decline even though the buyer changed the agreement by adding a shipping charge. I have spoken to two reps and a supervisor and in the end they said to wait for the buyer to open an unpaid item case and then dispute that. The buyer has not responded to my emails explaining why I want to cancel the transaction and Ebay says they will not cancel it, the buyer has to. All in all, they are saying that the seller can change the terms after the offer and there is nothing they can do about it. I tried to explain that this should be considered a counter offer since the terms were changed but it was like talking to a brick wall. In ten years on Ebay I have never seen or heard of this discussed anywhere but something has to be wrong here.
Thanks for letting me vent as this just has me in a tizzy and I do not have enough anxiety meds to use them on this.
B1ll D0nalds0n