Fix shipping fee calculations from United States Post Office

Require someone who has experience setting up the shipping system from the Unitied States Postal Service. API's have already been installed, but it is charging the customers too much. Also, we would like to have the Priority Mail Flat Rate Box option set up. Need to have this done in one day (2 days the most). Experience people only. Please provide websites where you set up the USPS system before. Thank you.