Extend the Jigoshop shipping methods by adding a basic bundle method.
This is a very simple plugin that allows you to set an initial shipping price for every product. You can set a smaller shipping price for subsequent products. The shipping price can be capped by setting a maximum number of products.
For example, the shop sells T-shirts. The shipping cost of one T-shirt is 2.50. For all other T-shirts ordered the shipping cost will be 1.00. However, the shop does not want to charge more that 6.50 in shipping for any given order. The product cap will therefore be five products: (1 x 2.50) + (4 x 1.00) = 6.50