Shipping Cost different at post office.

[steveq1701]

Hello Everyone, 

I just started selling on eBay and made my first two sales this week. I went through the process and printed my USPS shipping labels and went to the post office. Someone from customer service at eBay walked me through the process and we measured and weighed the box. When I went to the post office the clerk said the postage was wrong and charged me extra money and had to add a second shipping number. The second item was a smaller simpler package and when I went to the post office it was the same thing, wrong price and I had to pay more and get a second shipping number. I must be doing something wrong because I have been buying fromeBay for years and cant remember ever getting two shipping numbers. 

Any advice?