It sounds like you are not installing the new files onto your server ...

In your FTP program, you should see the directory:
/includes

for your shop ...

When you unzip the new USPS January 22, 2012 download, you should see the directory:
/usps_v15

If you go into that directory, you will see the directory:
/includes

and the file:
readme.txt

Now, you want to copy the directory:
/includes

to your server with your FTP program so that you are overwriting the files on your server:
/includes

If asked do you want to overwrite files, say YES ...

Next, follow the instructions in the readme.txt