Now I have one store and 2 different vendors. I have created an shipping extension similar to royal mail (This one for India mail). The shipping is weight based. Now everything works fine but when a buyer clicks both my product (less in weight) and the vendor product (more in weight) the calculation combines both weight and gives the flat shipping cost which is OK for the vendor but I loose my shipping cost. Currently I am handling this using ship product separately option. But the problem is for the buyer when they order 3 of my products and one of the vendor product the calculation shows 3 times the shipping cost for my product . Stuck big time.