Mailing a non-letter with Ebay Standard Envelope

Has anyone had an issue mailing a non-letter with postage paid through Ebay Standard Envelope? I sold a 1 oz coin which I thought was covered in this and put it in a 4" x 6" non-padded envelope and bought the postage through ebay for $0.84 (2 oz to be sure). I brought it to the post office to check if it was okay and the worker there said I will be charged another $4-5 saying you can only mail letters in an envelope. He rudely added that Ebay doesn't make the rules. I usually ship with boxes so this took me a bit by surprise and would like to hear other peoples thoughts on this. Thanks in advance