What is the cheapest way to mail/ship items in a large envelope? Does USPS charge more at the post office?

I have recently been selling on ebay but want to find the cheapest way to ship items. Today I checked online on USPS.com how much my package would cost me to send from PA to SC which weighed only 2 ounces (a small blouse in a large bubble mailer). Online said it would be $1.10. So when I went to the post office they charged me $2.30? Do they charge more when you take it to the post office? I know it's not that much but I ask the lady who took my package and I swear she was trying to go around my question and never answered me! I am thinking that maybe it was suppose to be $1.10 and she ripped me off and there is a way to ship it cheaper. If I just put stamps will it be cheaper or are there any other ways?? Thanks!