Shipping for additional products not getting added

When the user clicks "Continue Shopping" and adds additional products, the shipping charge does not change to reflect the additional products that have been added. Only the shipping for the first product added is shown. You can see this behavior at astragalus-source.com

Testing this issue further, I find that when I add an additional product with a different shipping rate than the first product, the additional cost for the shipping gets added properly. However if I add an additional product with the same shipping rate as the first product, the shipping rate remains unchanged, reflecting only the shipping for the first product (or the last product?)

OOPs I meant "shipping weight" not "shipping rate" in the above reply

Yes, that is my understanding of how it should work. To duplicate the problem, try this:
go to the order page and click on the second product on the list, Astragalus membranaceus granulated, add to cart, continue shopping. Go back to the order page and click on the third product on the list, Pure astragalus and centella asiatica blend, add to cart, continue shopping , return to order page. Click on the fourth product on the list, Pure astragalus membranaceus ground. Add to cart and proceed to checkout.

Each of these products has the shipping weight set as 0.5 kg, so there should be a total of 1.5kg in the cart. The shipping methods available for all countries that have been configured cover the range from 0.5kg to 3.99kg . The first weight range is 0.5kg to 0.99kg. The second range is 1.00kg to 1.49kg. The third range is 1.50kg to 1.99kg. Therefore with 1.5kg total in the cart it should show the third shipping method. If the country is left as the default France, the method for 1.5kg should be France - 3 and the shipping amount should be $57. When I try this, the shipping method it shows instead is France - 1 with a shipping amount of $45. Is my understanding not correct on this?

Also I wonder about your experience of getting the "Too large for any available shipping method error". The only way this should happen is if you added the heaviest product, the first one on the list, Pure astragalus membranaceus root - sliced, twice, as it has a shipping weight of 2.6kg. All the other products have a shipping weight of 0.5kg, so and combination of two different products should not exceed the 3.99kg method that is configured for all countries.

So sorry!! I entered all the product weights yesterday, I remember doing so distinctly, but today I find that only one product had the weight set. All I can figure is that I must not have been saving the changes properly. If you ever come to Cambodia, the drinks are on me!