Multicart Shipping Customization - repost

I have Iscripts Multicart 2.4 and need a shipping customization. The shipping methods that are currently installed are Fedex, UPS and USPS and they work fine. The program does allow each seller to offer their own flat rate shipping but the seller cannot put different flat rate shipping rates on each individual item they sell. Therefore the shipping rates are the same for all the seller’s products no matter if they are very large items or very small items. I need the shipping modified so that each seller can put their own different flat rate shipping rates on each item they sell. I do not want the current shipping methods removed, I just want to fix the flat rate shipping method to work better with the current shipping methods. Also the system is currently set up so that when a buyer has ordered products from multiple sellers on the same order, at checkout all the orders and shipping totals are calculated together so that the buyer can make one payment. Please make sure that if one or more of the sellers offers flat shipping and the others do not, on the “Select Shipping Method” page the Flat Rate Shipping, Fedex, UPS and USPS options are visible for the buyer to select and that the orders and the shipping rates from the different sellers are calculated together. If you need more information please contact me. Please only bid if you are serious doing the project. I have awarded this project before but the freelancer never did start the project or communicate with me. I have a small budget