Single charge for multiple orders via perwieightunit shipping

I've had a look through the forums and Zen Cart tutorials and not found the answer to my issue.

Basically I've set up perweightunit shipping option and everything works fine. The problem I have is that I want 1 charge per order so if a customer orders 3 items they are charged once at the rate of the most expensive item. At the moment it adds a charge per item.

Looking at the forums thus far I'm worried that a fix is based on editing php code, which is not something I'm overly familiar with. Any help would greatly be appreciated.

In terms of Zen Cart the store is running the latest version and has only the perweightunit shipping option enabled.

Ok I've looked at the perwieghtunit.php file located in includes>modules>shipping and noted that there are differences between this file and the one that ran on the previous version of the site (1.3.7 where the shipping option I mentioned above worked perfectly fine).

Having attempted modifying the file I get £0 shipping, which leads me to believe that perhaps a 2nd file has to be modified. Does anyone know if that is the case or am I simply not making the right changes to the perwieghtunit.php file?

Ok just a further update on this problem. I got a php coder to look at the file (includes>modules>shipping>perwieghtunit.php) and he made the necessary changes but still no joy. He suggested that there were other files that needed modification but without the familiarity with Zen Cart he wouldn't know which ones.

As a result I've simply opted for a table rate option, which pretty much matches my requirements.