Wow. It is the seller's responsibility to make sure that they have adequate shipping. If they sent it from their house, then I would question that they were accurate. If they sent it from a post office (from the counter or a meter machine), then I would definitely complain to my local post office and demand an explanation.

all I can think of that would cause that is if they used a Priority Mail envelope turned inside out.

That would do it, for sure. If you use anything that says, anyplace on it "priority mail" you'll get charged the priority rate, not even the flat rate envelope rate either.

I've heard that is true - even if the envelope or box is turned inside out.

There is no way an envelope that size and weight would cost that much, someone screwed up at the post office.

I sent out some fabric to someone in the US and it cost me 1.70. they measured the envelope in the post office to make sure it fit properly in their restrictions.

I wonder if they quoted you an expedited rate which would be more expensive. I'd try asking another post office.

If the envelope is more than 1/4" think it has to go parcel post. They have a cardboard thing that has a slot that the envelope has to pass through easily.

I would ask the e-bay seller for a refund of your postage paid when you tell her what you had to pay.

not unless it weighs more than 13 ounces. Up to 13 ounces - FIrst Class (there are 3 classes of First Class Mail). Over 13 ounces - Parcel Post or Priority.

I will go to post office 1st. I had already e-mailed sender
Wennine, she paid 1.75. I guess the rest of 3.50 is handling
for her.
It is not thick, envelope reg.manila almost 6 x almost 9. I guess from home meter- service on ebay. 3 0z. printed on envelope usps first class mail, eltronic rate approved on bottom. Then stamped balance due 5.31. I will get receipt tomorrow from payment to post office- rural mail.
I had guessed at envelope at first I measured.

YES...if it was more than 1/4" thick it is called a Parcel and costs like crazy...

what happens if I refuse postage, I don't get thr item and I don't think it was more than a 1/4".
have to try to get my money back. I" will let you know what happens at post office. It seems crazy, 3.50 paid by me to sender, and post office 5.31. I will find the answer I hope. For me that is 9.11 shipping. for a little, lite envelope