Is there a way to rework the code?

[marneo]

I'm using a PHP/MySQL shopping cart on my website called Squirrelcart and I've just run into a problem which had not come to my attention before and I would greatly appreciate any suggestions as to how to alter the code to fix this glitch. 

Squirrelcart supports real time rates from both UPS and FedEx but it turns out now that you cannot assign one shipper to a given product to the exclusion of another.  A more detailed description of the problem which has arisen is as follows:

We work with drop shippers.  Up until now, the drop shippers we were working with all used UPS so we've had UPS Online Tools enabled on our site from the beginning to return real time UPS rates to our customers.  Now we've got a new drop shipper and he is the first to use FedEx (and not UPS) so we have had our first occasion to enable FedEx Ship Manager Direct on the site but obviously we want to segregate the items which ship via UPS from those which ship via FedEx.  This was the first time it came to our attention that this is not possible given the way Squirrelcart is designed. 

In configuring 'Product Details', 'Rates & Services', 'Ship Using' only offers a choice between 'enabled services' and 'a specific service' so that you cannot choose 'UPS enabled services' or 'FedEx enabled services'.  I have all of both UPS's and FedEx's services enabled in their respective shipping modules in the control panel and I want to be able to allow the customer to be able to choose whichever option he prefers - Ground, Overnight, etc. as offered by that courier but I can't have customers choosing UPS for items whose drop shipper only uses FedEx and FedEx for items whose drop shipper only uses UPS and right now, that is the way the checkout page is appearing with a choice between the 2 couriers available for products that can only be shipped by one courier as designated by the drop shopper.  Is there a way to rework the code so that only FedEx will appear for cetain products and only UPS for others?'