Fix oscommerce site bug/ basic site work

One of my coders created these two sites for me using oscommerce: [login to view URL] [login to view URL] The shipping estimator gives a different price then the shopping cart page, which gives a different price than the checkout page. The coder I hired wasn't able to fix this problem. I want to fix this problem by making shipping costs the exact same throughout the customer checkout experience. Please note this fix needs to be applied to BOTH websites. Google checkout, paypal, shipping estimator, etc. all need to show the SAME shipping price. This shipping price needs to be ACCURATE to the real-life shipping costs which UPS has. I need to be able to control whether or not a part can be combined with other parts when estimating shipping weight and thus shipping costs. I need to have the ability to add product dimensions for each part within the kully solar shop. Because solar panels are often over-sized, I need to account for this. I need to be able to account for freight shipping and account for pallets. I need the coder to add a way to increase the cost of oversized items like solar panels appropriately and with ease. Needs to be a simple one or two question prompt which automatically estimates the freight shipping cost ACCURATELY. Also, I want the checkout experience to be one page. Right now the user has to push the continue button numerous times.. I want it to be a breeze just like newegg.com. I want the checkout page to look nearly identical to, without copywrite infringrement, [login to view URL]'s checkout page. One page, clean, and FUNCTIONAL! FULL BACKUP MUST BE PERFORMED BEFORE WORKING ON BOTH PROJECTS!