Calculating Shipping Postage

[myowntracks21]

I purchased the shipping label, and printed it off. 2.67, I believe. When I went to the USPS, they weighed it, and it calculated more. So, she couldn't refund any money, as she didn't know how much I had purchased the label from eBay for. I told her. But, she said, there was no proof. So, I had to weigh it with her, and get a "new label", with a new tracking number! Don't know what I did wrong, but I had to pay,,,,,,,, again,,,,, 2.67. Lol, so, I need help. As, I am new to eBay!