For one time products, a shipping calculator which charged the price per weight would be helpful. It should also be smart enough to combine the weight of 2 products being purchased at once to calculate the shipping.
For example: Customer buys a book at 2.5lbs and a toy at .75 lbs. The website would calculate shipping based on a package of 3.25lbs
Most subscription managers and e-commerce tools has the ability to have real-time or dynamic shipping charges based on shipping carrier, weight and where the item is being shipped to. I would prefer not to have a flat shipping fee for all subscribers or include the cost of shipping in the cost of the product.