Shipping Module Configuration failure

[whataxiao]

I have USPS username and password. When i go to  Shipping Module List enter the username and password then hit save.
It came out

Error: Unable to write into the configuration file /var/www/vhosts/MYWEBSITE/httpdocs/administrator/components/com_virtuemart/classes/payment/.cfg.php!

do i need to install USPS module or the virtuemart already have one ?

[lowmips]

Which module are you attempting to configure? It's writing to a very strange location...

[whataxiao]

USPS module Configure Ship Method 3.0 (2007/06/21)   

[lowmips]

That's an old version of the module.  There's no reason to fix your issue with that version. With the recent Jan 2 USPS service name changes, it will not work anyway once you've fixed the configuration error. 

You'd be better off with Chaz's version 4.2 or my version 5 USPS modules.
Chaz's USPS 4.2 module is here: http://forum.virtuemart.net/index.php?topic=40284.0
My USPSv5 module is here: http://forum.virtuemart.net/index.php?topic=46784.0